A Guide to Constructors

Issue: BAEL-2345

core-java/src/main/java/com/baeldung/constructors/BankAccount.java

@@ -0,0 +1,56 @@
+package com.baeldung.constructors;
+
+import java.time.LocalDateTime;
+
+class BankAccount {
+    String name;
+    LocalDateTime opened;
+    double balance;
+    
+    @Override
+    public String toString() {
+        return String.format("%s, %s, %f", this.name, this.opened.toString(), this.balance);
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public LocalDateTime getOpened() {
+        return opened;
+    }
+
+    public double getBalance() {
+        return this.balance;
+    }
+}
+
+class BankAccountEmptyConstructor extends BankAccount {
+    public BankAccountEmptyConstructor() {
+        this.name = "";
+        this.opened = LocalDateTime.now();
+        this.balance = 0.0d;
+    }
+}
+
+class BankAccountParameterizedConstructor extends BankAccount {
+    public BankAccountParameterizedConstructor(String name, LocalDateTime opened, double balance) {
+        this.name = name;
+        this.opened = opened;
+        this.balance = balance;
+    }
+}
+
+class BankAccountCopyConstructor extends BankAccount {
+    public BankAccountCopyConstructor(String name, LocalDateTime opened, double balance) {
+        this.name = name;
+        this.opened = opened;
+        this.balance = balance;
+    }
+        
+    public BankAccountCopyConstructor(BankAccount other) {
+        this.name = other.name;
+        this.opened = LocalDateTime.now();
+        this.balance = 0.0f;
+    }
+}

core-java/src/main/java/com/baeldung/constructors/Transaction.java

@@ -0,0 +1,25 @@
+package com.baeldung.constructors;
+
+import java.time.LocalDateTime;
+
+class Transaction {
+    final BankAccountEmptyConstructor bankAccount;
+    final LocalDateTime date;
+    final double amount;
+
+    public Transaction(BankAccountEmptyConstructor account, LocalDateTime date, double amount) {
+        this.bankAccount = account;
+        this.date = date;
+        this.amount = amount;
+    }
+
+    /*
+     * Compilation Error :'(, all final variables must be explicitly initialised.
+     * public Transaction() {
+     * }
+     */
+
+    public void invalidMethod() {
+        // this.amount = 102.03; // Results in a compiler error. You cannot change the value of a final variable.
+    }    
+}

core-java/src/test/java/com/baeldung/constructors/ConstructorUnitTest.java

@@ -0,0 +1,53 @@
+package com.baeldung.constructors;
+
+import com.baeldung.constructors.*;
+
+import java.util.logging.Logger;
+import java.time.LocalDateTime;
+import java.time.Month;
+
+import org.junit.Test;
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatCode;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+public class ConstructorUnitTest {
+    final static Logger LOGGER = Logger.getLogger(ConstructorUnitTest.class.getName());
+
+    @Test
+    public void givenNoExplicitContructor_whenUsed_thenFails() {
+        BankAccount account = new BankAccount();
+        assertThatThrownBy(() -> { account.toString(); }).isInstanceOf(Exception.class);
+    }
+
+    @Test
+    public void givenNoArgumentConstructor_whenUsed_thenSucceeds() {
+        BankAccountEmptyConstructor account = new BankAccountEmptyConstructor();
+        assertThatCode(() -> {
+                account.toString();
+            }).doesNotThrowAnyException();
+    }
+
+    @Test
+    public void givenParameterisedConstructor_whenUsed_thenSucceeds() {
+        LocalDateTime opened = LocalDateTime.of(2018, Month.JUNE, 29, 06, 30, 00);
+        BankAccountParameterizedConstructor account =
+            new BankAccountParameterizedConstructor("Tom", opened, 1000.0f);
+        
+        assertThatCode(() -> {
+                account.toString();
+            }).doesNotThrowAnyException();
+    }
+
+    @Test
+    public void givenCopyContructor_whenUser_thenMaintainsLogic() {
+        LocalDateTime opened = LocalDateTime.of(2018, Month.JUNE, 29, 06, 30, 00);
+        BankAccountCopyConstructor account = new BankAccountCopyConstructor("Tim", opened, 1000.0f);
+        BankAccountCopyConstructor newAccount = new BankAccountCopyConstructor(account);
+
+        assertThat(account.getName()).isEqualTo(newAccount.getName());
+        assertThat(account.getOpened()).isNotEqualTo(newAccount.getOpened());
+        
+        assertThat(newAccount.getBalance()).isEqualTo(0.0f);
+    }
+}