[[painless-operators-reference]]
=== Operators: Reference

[[method-call-operator]]
==== Method Call

Use the `method call operator '()'` to call a member method on a
<<reference-types, reference type>> value. Implicit
<<boxing-unboxing, boxing/unboxing>> is evaluated as necessary per argument
during the method call. When a method call is made on a target `def` type value,
the parameters and return type value are considered to also be of the `def` type
and are evaluated at run-time.

An overloaded method is one that shares the same name with two or more methods.
A method is overloaded based on arity where the same name is re-used for
multiple methods as long as the number of parameters differs.

*Errors*

* If the reference type value is `null`.
* If the member method name doesn't exist for a given reference type value.
* If the number of arguments passed in is different from the number of specified
  parameters.
* If the arguments cannot be implicitly cast or implicitly boxed/unboxed to the
  correct type values for the parameters.

*Grammar*

[source,ANTLR4]
----
method_call: '.' ID arguments;
arguments: '(' (expression (',' expression)*)? ')';
----

*Examples*

* Method calls on different reference types.
+
[source,Painless]
----
Map m = new HashMap();                         <1>
m.put(1, 2);                                   <2>
int z = m.get(1);                              <3>
def d = new ArrayList();                       <4>
d.add(1);                                      <5>
int i = Integer.parseInt(d.get(0).toString()); <6>
----
+
<1> declare `Map m`;
    allocate `HashMap` instance -> `HashMap reference`;
    store `HashMap reference` to `m`
<2> load from `m` -> `Map reference`;
    implicit cast `int 1` to `def` -> `def`;
    implicit cast `int 2` to `def` -> `def`;
    call `put` on `Map reference` with arguments (`int 1`, `int 2`)
<3> declare `int z`;
    load from `m` -> `Map reference`;
    call `get` on `Map reference` with arguments (`int 1`) -> `def`;
    implicit cast `def` to `int 2` -> `int 2`;
    store `int 2` to `z`
<4> declare `def d`;
    allocate `ArrayList` instance -> `ArrayList reference`;
    implicit cast `ArrayList` to `def` -> `def`;
    store `def` to `d`
<5> load from `d` -> `def`;
    implicit cast `def` to `ArrayList reference` -> `ArrayList reference`
    call `add` on `ArrayList reference` with arguments (`int 1`);
<6> declare `int i`;
    load from `d` -> `def`;
    implicit cast `def` to `ArrayList reference` -> `ArrayList reference`
    call `get` on `ArrayList reference` with arguments (`int 1`) -> `def`;
    implicit cast `def` to `Integer 1 reference` -> `Integer 1 reference`;
    call `toString` on `Integer 1 reference` -> `String '1'`;
    call `parseInt` on `Integer` with arguments (`String '1'`) -> `int 1`;
    store `int 1` in `i`;

[[field-access-operator]]
==== Field Access

Use the `field access operator '.'` to store a value to or load a value from a
<<reference-types, reference type>> member field.

*Errors*

* If the reference type value is `null`.
* If the member field name doesn't exist for a given reference type value.

*Grammar*

[source,ANTLR4]
----
field_access: '.' ID;
----

*Examples*

The examples use the following reference type definition:

[source,Painless]
----
name:
  Example

non-static member fields:
  * int x
  * def y
  * List z
----

* Field access with the `Example` type.
+
[source,Painless]
----
Example example = new Example(); <1>
example.x = 1;                   <2>
example.y = example.x;           <3>
example.z = new ArrayList();     <4>
example.z.add(1);                <5>
example.x = example.z.get(0);    <6>
----
+
<1> declare `Example example`;
    allocate `Example` instance -> `Example reference`;
    store `Example reference` to `example`
<2> load from `example` -> `Example reference`;
    store `int 1` to `x` of `Example reference`
<3> load from `example` -> `Example reference @0`;
    load from `example` -> `Example reference @1`;
    load from `x` of `Example reference @1` -> `int 1`;
    implicit cast `int 1` to `def` -> `def`;
    store `def` to `y` of `Example reference @0`;
    (note `Example reference @0` and `Example reference @1` are the same)
<4> load from `example` -> `Example reference`;
    allocate `ArrayList` instance -> `ArrayList reference`;
    implicit cast `ArrayList reference` to `List reference` -> `List reference`;
    store `List reference` to `z` of `Example reference`
<5> load from `example` -> `Example reference`;
    load from `z` of `Example reference` -> `List reference`;
    call `add` on `List reference` with arguments (`int 1`)
<6> load from `example` -> `Example reference @0`;
    load from `example` -> `Example reference @1`;
    load from `z` of `Example reference @1` -> `List reference`;
    call `get` on `List reference` with arguments (`int 0`) -> `int 1`;
    store `int 1` in `x` of `List reference @0`;
    (note `Example reference @0` and `Example reference @1` are the same)

[[null-safe-operator]]
==== Null Safe

Use the `null safe operator '?.'` instead of the method call operator or field
access operator to ensure a reference type value is `non-null` before
a method call or field access. A `null` value will be returned if the reference
type value is `null`, otherwise the method call or field access is evaluated.

*Errors*

* If the method call return type value or the field access type value is not
  a reference type value and is not implicitly castable to a reference type
  value.

*Grammar*

[source,ANTLR4]
----
null_safe: null_safe_method_call
         | null_safe_field_access
         ;

null_safe_method_call: '?.' ID arguments;
arguments: '(' (expression (',' expression)*)? ')';

null_safe_field_access: '?.' ID;
----

*Examples*

The examples use the following reference type definition:

[source,Painless]
----
name:
  Example

non-static member methods:
  * List factory()

non-static member fields:
  * List x
----

* Null safe without a `null` value.
+
[source,Painless]
----
Example example = new Example(); <1>
List x = example?.factory();     <2>
----
+
<1> declare `Example example`;
    allocate `Example` instance -> `Example reference`;
    store `Example reference` to `example`
<2> declare `List x`;
    load from `example` -> `Example reference`;
    null safe call `factory` on `Example reference` -> `List reference`;
    store `List reference` to `x`;
+
* Null safe with a `null` value;
+
[source,Painless]
----
Example example = null; <1>
List x = example?.x;    <2>
----
<1> declare `Example example`;
    store `null` to `example`
<2> declare `List x`;
    load from `example` -> `Example reference`;
    null safe access `x` on `Example reference` -> `null`;
    store `null` to `x`;
    (note the *null safe operator* returned `null` because `example` is `null`)

[[list-initialization-operator]]
==== List Initialization

Use the `list initialization operator '[]'` to allocate an `List` type instance
to the heap with a set of pre-defined values. Each value used to initialize the
`List` type instance is cast to a `def` type value upon insertion into the
`List` type instance using the `add` method. The order of the specified values
is maintained.

*Grammar*

[source,ANTLR4]
----
list_initialization: '[' expression (',' expression)* ']'
                   | '[' ']';
----

*Examples*

* List initialization of an empty `List` type value.
+
[source,Painless]
----
List empty = []; <1>
----
+
<1> declare `List empty`;
    allocate `ArrayList` instance -> `ArrayList reference`;
    implicit cast `ArrayList reference` to `List reference` -> `List reference`;
    store `List reference` to `empty`
+
* List initialization with static values.
+
[source,Painless]
----
List list = [1, 2, 3]; <1>
----
+
<1> declare `List list`;
    allocate `ArrayList` instance -> `ArrayList reference`;
    call `add` on `ArrayList reference` with arguments(`int 1`);
    call `add` on `ArrayList reference` with arguments(`int 2`);
    call `add` on `ArrayList reference` with arguments(`int 3`);
    implicit cast `ArrayList reference` to `List reference` -> `List reference`;
    store `List reference` to `list`
+
* List initialization with non-static values.
+
[source,Painless]
----
int i = 1;                  <1>
long l = 2L;                <2>
float f = 3.0F;             <3>
double d = 4.0;             <4>
String s = "5";             <5>
List list = [i, l, f*d, s]; <6>
----
+
<1> declare `int i`;
    store `int 1` to `i`
<2> declare `long l`;
    store `long 2` to `l`
<3> declare `float f`;
    store `float 3.0` to `f`
<4> declare `double d`;
    store `double 4.0` to `d`
<5> declare `String s`;
    store `String "5"` to `s`
<6> declare `List list`;
    allocate `ArrayList` instance -> `ArrayList reference`;
    load from `i` -> `int 1`;
    call `add` on `ArrayList reference` with arguments(`int 1`);
    load from `l` -> `long 2`;
    call `add` on `ArrayList reference` with arguments(`long 2`);
    load from `f` -> `float 3.0`;
    load from `d` -> `double 4.0`;
    promote `float 3.0` and `double 4.0`: result `double`;
    implicit cast `float 3.0` to `double 3.0` -> `double 3.0`;
    multiply `double 3.0` and `double 4.0` -> `double 12.0`;
    call `add` on `ArrayList reference` with arguments(`double 12.0`);
    load from `s` -> `String "5"`;
    call `add` on `ArrayList reference` with arguments(`String "5"`);
    implicit cast `ArrayList reference` to `List reference` -> `List reference`;
    store `List reference` to `list`

[[list-access-operator]]
==== List Access

Use the `list access operator '[]'` as a shortcut for a `set` method call or
`get` method call made on a `List` type value.

*Errors*

* If a value other than a `List` type value is accessed.
* If a non-integer type value is used as an index for a `set` method call or
  `get` method call.

*Grammar*

[source,ANTLR4]
----
list_access: '[' expression ']'
----

*Examples*

* List access with the `List` type.
+
[source,Painless]
----
List list = new ArrayList(); <1>
list.add(1);                 <2>
list.add(2);                 <3>
list.add(3);                 <4>
list[0] = 2;                 <5>
list[1] = 5;                 <6>
int x = list[0] + list[1];   <7>
int y = 1;                   <8>
int z = list[y];             <9>
----
+
<1> declare `List list`;
    allocate `ArrayList` instance -> `ArrayList reference`;
    implicit cast `ArrayList reference` to `List reference` -> `List reference`;
    store `List reference` to `list`
<2> load from `list` -> `List reference`;
    call `add` on `List reference` with arguments(`int 1`)
<3> load from `list` -> `List reference`;
    call `add` on `List reference` with arguments(`int 2`)
<4> load from `list` -> `List reference`;
    call `add` on `List reference` with arguments(`int 3`)
<5> load from `list` -> `List reference`;
    call `set` on `List reference` with arguments(`int 0`, `int 2`)
<6> load from `list` -> `List reference`;
    call `set` on `List reference` with arguments(`int 1`, `int 5`)
<7> declare `int x`;
    load from `list` -> `List reference`;
    call `get` on `List reference` with arguments(`int 0`) -> `def`;
    implicit cast `def` to `int 2` -> `int 2`;
    load from `list` -> `List reference`;
    call `get` on `List reference` with arguments(`int 1`) -> `def`;
    implicit cast `def` to `int 5` -> `int 5`;
    add `int 2` and `int 5` -> `int 7`;
    store `int 7` to `x`
<8> declare `int y`;
    store `int 1` int `y`
<9> declare `int z`;
    load from `list` -> `List reference`;
    load from `y` -> `int 1`;
    call `get` on `List reference` with arguments(`int 1`) -> `def`;
    implicit cast `def` to `int 5` -> `int 5`;
    store `int 5` to `z`
+
* List access with the `def` type.
+
[source,Painless]
----
def d = new ArrayList(); <1>
d.add(1);                <2>
d.add(2);                <3>
d.add(3);                <4>
d[0] = 2;                <5>
d[1] = 5;                <6>
def x = d[0] + d[1];     <7>
def y = 1;               <8>
def z = d[y];            <9>
----
+
<1> declare `List d`;
    allocate `ArrayList` instance -> `ArrayList reference`;
    implicit cast `ArrayList reference` to `def` -> `def`;
    store `def` to `d`
<2> load from `d` -> `def`;
    implicit cast `def` to `ArrayList reference` -> `ArrayList reference`;
    call `add` on `ArrayList reference` with arguments(`int 1`)
<3> load from `d` -> `def`;
    implicit cast `def` to `ArrayList reference` -> `ArrayList reference`;
    call `add` on `ArrayList reference` with arguments(`int 2`)
<4> load from `d` -> `def`;
    implicit cast `def` to `ArrayList reference` -> `ArrayList reference`;
    call `add` on `ArrayList reference` with arguments(`int 3`)
<5> load from `d` -> `def`;
    implicit cast `def` to `ArrayList reference` -> `ArrayList reference`;
    call `set` on `ArrayList reference` with arguments(`int 0`, `int 2`)
<6> load from `d` -> `def`;
    implicit cast `def` to `ArrayList reference` -> `ArrayList reference`;
    call `set` on `ArrayList reference` with arguments(`int 1`, `int 5`)
<7> declare `def x`;
    load from `d` -> `def`;
    implicit cast `def` to `ArrayList reference` -> `ArrayList reference`;
    call `get` on `ArrayList reference` with arguments(`int 0`) -> `def`;
    implicit cast `def` to `int 2` -> `int 2`;
    load from `d` -> `def`;
    implicit cast `def` to `ArrayList reference` -> `ArrayList reference`;
    call `get` on `ArrayList reference` with arguments(`int 1`) -> `def`;
    implicit cast `def` to `int 2` -> `int 2`;
    add `int 2` and `int 5` -> `int 7`;
    store `int 7` to `x`
<8> declare `int y`;
    store `int 1` int `y`
<9> declare `int z`;
    load from `d` -> `ArrayList reference`;
    load from `y` -> `def`;
    implicit cast `def` to `int 1` -> `int 1`;
    call `get` on `ArrayList reference` with arguments(`int 1`) -> `def`;
    store `def` to `z`

[[map-initialization-operator]]
==== Map Initialization

Use the `map initialization operator '[:]'` to allocate a `Map` type instance to
the heap with a set of pre-defined values. Each pair of values used to
initialize the `Map` type instance are cast to `def` type values upon insertion
into the `Map` type instance using the `put` method.

*Grammar*

[source,ANTLR4]
----
map_initialization: '[' key_pair (',' key_pair)* ']'
                  | '[' ':' ']';
key_pair: expression ':' expression
----

*Examples*

* Map initialization of an empty `Map` type value.
+
[source,Painless]
----
Map empty = [:]; <1>
----
+
<1> declare `Map empty`;
    allocate `HashMap` instance -> `HashMap reference`;
    implicit cast `HashMap reference` to `Map reference` -> `Map reference`;
    store `Map reference` to `empty`
+
* Map initialization with static values.
+
[source,Painless]
----
Map map = [1:2, 3:4, 5:6]; <1>
----
+
<1> declare `Map map`;
    allocate `HashMap` instance -> `HashMap reference`;
    call `put` on `HashMap reference` with arguments(`int 1`, `int 2`);
    call `put` on `HashMap reference` with arguments(`int 3`, `int 4`);
    call `put` on `HashMap reference` with arguments(`int 5`, `int 6`);
    implicit cast `HashMap reference` to `Map reference` -> `Map reference`;
    store `Map reference` to `map`
+
* Map initialization with non-static values.
+
[source,Painless]
----
byte b = 0;                  <1>
int i = 1;                   <2>
long l = 2L;                 <3>
float f = 3.0F;              <4>
double d = 4.0;              <5>
String s = "5";              <6>
Map map = [b:i, l:f*d, d:s]; <7>
----
+
<1> declare `byte b`;
    store `byte 0` to `b`
<2> declare `int i`;
    store `int 1` to `i`
<3> declare `long l`;
    store `long 2` to `l`
<4> declare `float f`;
    store `float 3.0` to `f`
<5> declare `double d`;
    store `double 4.0` to `d`
<6> declare `String s`;
    store `String "5"` to `s`
<7> declare `Map map`;
    allocate `HashMap` instance -> `HashMap reference`;
    load from `b` -> `byte 0`;
    load from `i` -> `int 1`;
    call `put` on `HashMap reference` with arguments(`byte 0`, `int 1`);
    load from `l` -> `long 2`;
    load from `f` -> `float 3.0`;
    load from `d` -> `double 4.0`;
    promote `float 3.0` and `double 4.0`: result `double`;
    implicit cast `float 3.0` to `double 3.0` -> `double 3.0`;
    multiply `double 3.0` and `double 4.0` -> `double 12.0`;
    call `put` on `HashMap reference` with arguments(`long 2`, `double 12.0`);
    load from `d` -> `double 4.0`;
    load from `s` -> `String "5"`;
    call `put` on `HashMap reference` with
            arguments(`double 4.0`, `String "5"`);
    implicit cast `HashMap reference` to `Map reference` -> `Map reference`;
    store `Map reference` to `map`

[[map-access-operator]]
==== Map Access

Use the `map access operator '[]'` as a shortcut for a `put` method call or
`get` method call made on a `Map` type value.

*Errors*

* If a value other than a `Map` type value is accessed.

*Grammar*
[source,ANTLR4]
----
map_access: '[' expression ']'
----

*Examples*

* Map access with the `Map` type.
+
[source,Painless]
----
Map map = new HashMap();               <1>
map['value2'] = 2;                     <2>
map['value5'] = 5;                     <3>
int x = map['value2'] + map['value5']; <4>
String y = 'value5';                   <5>
int z = x[z];                          <6>
----
+
<1> declare `Map map`;
    allocate `HashMap` instance -> `HashMap reference`;
    implicit cast `HashMap reference` to `Map reference` -> `Map reference`;
    store `Map reference` to `map`
<2> load from `map` -> `Map reference`;
    call `put` on `Map reference` with arguments(`String 'value2'`, `int 2`)
<3> load from `map` -> `Map reference`;
    call `put` on `Map reference` with arguments(`String 'value5'`, `int 5`)
<4> declare `int x`;
    load from `map` -> `Map reference`;
    call `get` on `Map reference` with arguments(`String 'value2'`) -> `def`;
    implicit cast `def` to `int 2` -> `int 2`;
    load from `map` -> `Map reference`;
    call `get` on `Map reference` with arguments(`String 'value5'`) -> `def`;
    implicit cast `def` to `int 5` -> `int 5`;
    add `int 2` and `int 5` -> `int 7`;
    store `int 7` to `x`
<5> declare `String y`;
    store `String 'value5'` to `y`
<6> declare `int z`;
    load from `map` -> `Map reference`;
    load from `y` -> `String 'value5'`;
    call `get` on `Map reference` with arguments(`String 'value5'`) -> `def`;
    implicit cast `def` to `int 5` -> `int 5`;
    store `int 5` to `z`
+
* Map access with the `def` type.
+
[source,Painless]
----
def d = new HashMap();             <1>
d['value2'] = 2;                   <2>
d['value5'] = 5;                   <3>
int x = d['value2'] + d['value5']; <4>
String y = 'value5';               <5>
def z = d[y];                      <6>
----
+
<1> declare `def d`;
    allocate `HashMap` instance -> `HashMap reference`;
    implicit cast `HashMap reference` to `def` -> `def`;
    store `def` to `d`
<2> load from `d` -> `def`;
    implicit cast `def` to `HashMap reference` -> `HashMap reference`;
    call `put` on `HashMap reference` with arguments(`String 'value2'`, `int 2`)
<3> load from `d` -> `def`;
    implicit cast `def` to `HashMap reference` -> `HashMap reference`;
    call `put` on `HashMap reference` with arguments(`String 'value5'`, `int 5`)
<4> declare `int x`;
    load from `d` -> `def`;
    implicit cast `def` to `HashMap reference` -> `HashMap reference`;
    call `get` on `HashMap reference` with arguments(`String 'value2'`)
            -> `def`;
    implicit cast `def` to `int 2` -> `int 2`;
    load from `d` -> `def`;
    call `get` on `HashMap reference` with arguments(`String 'value5'`)
            -> `def`;
    implicit cast `def` to `int 5` -> `int 5`;
    add `int 2` and `int 5` -> `int 7`;
    store `int 7` to `x`
<5> declare `String y`;
    store `String 'value5'` to `y`
<6> declare `def z`;
    load from `d` -> `def`;
    load from `y` -> `String 'value5'`;
    call `get` on `HashMap reference` with arguments(`String 'value5'`)
            -> `def`;
    store `def` to `z`

[[new-instance-operator]]
==== New Instance

Use the `new instance operator 'new ()'` to allocate a
<<reference-types, reference type>> instance to the heap and call a specified
constructor. Implicit <<boxing-unboxing, boxing/unboxing>> is evaluated as
necessary per argument during the constructor call.

An overloaded constructor is one that shares the same name with two or more
constructors. A constructor is overloaded based on arity where the same
reference type name is re-used for multiple constructors as long as the number
of parameters differs.

*Errors*

* If the reference type name doesn't exist for instance allocation.
* If the number of arguments passed in is different from the number of specified
  parameters.
* If the arguments cannot be implicitly cast or implicitly boxed/unboxed to the
  correct type values for the parameters.

*Grammar*

[source,ANTLR4]
----
new_instance: 'new' TYPE '(' (expression (',' expression)*)? ')';
----

*Examples*

* Allocation of new instances with different types.

[source,Painless]
----
Map m = new HashMap();   <1>
def d = new ArrayList(); <2>
def e = new HashMap(m);  <3>
----
<1> declare `Map m`;
    allocate `HashMap` instance -> `HashMap reference`;
    implicit cast `HashMap reference` to `Map reference` -> `Map reference`;
    store `Map reference` to `m`;
<2> declare `def d`;
    allocate `ArrayList` instance -> `ArrayList reference`;
    implicit cast `ArrayList reference` to `def` -> `def`;
    store `def` to `d`;
<3> declare `def e`;
    load from `m` -> `Map reference`;
    allocate `HashMap` instance with arguments (`Map reference`)
            -> `HashMap reference`;
    implicit cast `HashMap reference` to `def` -> `def`;
    store `def` to `e`;

[[string-concatenation-operator]]
==== String Concatenation

Use the `string concatenation operator '+'` to concatenate two values together
where at least one of the values is a <<string-type, `String` type>>.

*Grammar*

[source,ANTLR4]
----
concatenate: expression '+' expression;
----

*Examples*

* String concatenation with different primitive types.
+
[source,Painless]
----
String x = "con";     <1>
String y = x + "cat"; <2>
String z = 4 + 5 + x; <3>
----
+
<1> declare `String x`;
    store `String "con"` to `x`;
<2> declare `String y`;
    load from `x` -> `String "con"`;
    concat `String "con"` and `String "cat"` -> `String "concat"`;
    store `String "concat"` to `y`
<3> declare `String z`;
    add `int 4` and `int 5` -> `int 9`;
    concat `int 9` and `String "9concat"`;
    store `String "9concat"` to `z`;
    (note the addition is done prior to the concatenation due to precedence and
            associativity of the specific operations)
+
* String concatenation with the `def` type.
+
[source,Painless]
----
def d = 2;             <1>
d = "con" + d + "cat"; <2>
----
+
<1> declare `def`;
    implicit cast `int 2` to `def` -> `def`;
    store `def` in `d`;
<2> concat `String "con"` and `int 9` -> `String "con9"`;
    concat `String "con9"` and `String "con"` -> `String "con9cat"`
    implicit cast `String "con9cat"` to `def` -> `def`;
    store `def` to `d`;
    (note the switch in type of `d` from `int` to `String`)

[[elvis-operator]]
==== Elvis

An elvis consists of two expressions. The first expression is evaluated
with to check for a `null` value. If the first expression evaluates to
`null` then the second expression is evaluated and its value used. If the first
expression evaluates to `non-null` then the resultant value of the first
expression is used. Use the `elvis operator '?:'` as a shortcut for the
conditional operator.

*Errors*

* If the first expression or second expression cannot produce a `null` value.

*Grammar*

[source,ANTLR4]
----
elvis: expression '?:' expression;
----

*Examples*

* Elvis with different reference types.
+
[source,Painless]
----
List x = new ArrayList();      <1>
List y = x ?: new ArrayList(); <2>
y = null;                      <3>
List z = y ?: new ArrayList(); <4>
----
+
<1> declare `List x`;
    allocate `ArrayList` instance -> `ArrayList reference`;
    implicit cast `ArrayList reference` to `List reference` -> `List reference`;
    store `List reference` to `x`;
<2> declare `List y`;
    load `x` -> `List reference`;
    `List reference` equals `null` -> `false`;
    evaluate 1st expression: `List reference` -> `List reference`;
    store `List reference` to `y`
<3> store `null` to `y`;
<4> declare `List z`;
    load `y` -> `List reference`;
    `List reference` equals `null` -> `true`;
    evaluate 2nd expression:
            allocate `ArrayList` instance -> `ArrayList reference`;
    implicit cast `ArrayList reference` to `List reference` -> `List reference`;
    store `List reference` to `z`;
