–BAEL-4986: Private constructors in Java

core-java-modules/core-java-lang-oop-constructors/src/main/java/com/baeldung/privateconstructors/Employee.java

@@ -0,0 +1,39 @@
+package com.baeldung.privateconstructors;
+
+public class Employee {
+
+    private final String name;
+    private final int age;
+    private final String department;
+
+    private Employee(String name, int age, String department) {
+        this.name = name;
+        this.age = age;
+        this.department = department;
+    }
+
+    public static class Builder {
+        private String name;
+        private int age;
+        private String department;
+
+        public Builder setName(String name) {
+            this.name = name;
+            return this;
+        }
+
+        public Builder setAge(int age) {
+            this.age = age;
+            return this;
+        }
+
+        public Builder setDepartment(String department) {
+            this.department = department;
+            return this;
+        }
+
+        public Employee build() {
+            return new Employee(name, age, department);
+        }
+    }
+}

core-java-modules/core-java-lang-oop-constructors/src/main/java/com/baeldung/privateconstructors/PrivateConstructorClass.java

@@ -0,0 +1,8 @@
+package com.baeldung.privateconstructors;
+
+public class PrivateConstructorClass {
+
+    private PrivateConstructorClass() {
+        System.out.println("In the private constructor");
+    }
+}

core-java-modules/core-java-lang-oop-constructors/src/main/java/com/baeldung/privateconstructors/SingletonClass.java

@@ -0,0 +1,20 @@
+package com.baeldung.privateconstructors;
+
+public final class SingletonClass {
+
+    private static SingletonClass INSTANCE;
+    private String info = "Initial info class";
+
+    private SingletonClass() {
+    }
+
+    public static SingletonClass getInstance() {
+        if (INSTANCE == null) {
+            INSTANCE = new SingletonClass();
+        }
+
+        return INSTANCE;
+    }
+
+    // getters and setters
+}

core-java-modules/core-java-lang-oop-constructors/src/main/java/com/baeldung/privateconstructors/StringUtils.java

@@ -0,0 +1,16 @@
+package com.baeldung.privateconstructors;
+
+public class StringUtils {
+
+    private StringUtils() {
+        System.out.println("This class cannot be instantiated");
+    }
+
+    public static String toUpperCase(String s) {
+        return s.toUpperCase();
+    }
+
+    public static String toLowerCase(String s) {
+        return s.toLowerCase();
+    }
+}

core-java-modules/core-java-lang-oop-constructors/src/main/java/com/baeldung/privateconstructors/ValueTypeClass.java

@@ -0,0 +1,22 @@
+package com.baeldung.privateconstructors;
+
+public class ValueTypeClass {
+
+    private final String value;
+    private final String type;
+
+    public ValueTypeClass(int x) {
+        this(Integer.toString(x), "int");
+    }
+
+    public ValueTypeClass(boolean x) {
+        this(Boolean.toString(x), "boolean");
+    }
+
+    private ValueTypeClass(String value, String type) {
+        this.value = value;
+        this.type = type;
+    }
+
+    // getters and setters
+}