BankAccount DCI example in Java

BankAccount.java

package org.kt3k.bankaccount;

public class BankAccount {

    private String id;
    private Integer balance;

    public BankAccount(String id, Integer balance) {
        this.id = id;
        this.balance = balance;
    }


    public void increase(Integer money) {
        this.balance += money;
    }

    public void decrease(Integer money) {
        this.balance -= money;
    }

    public Integer getBalance() {
        return this.balance;
    }

    public String getId() {
        return this.id;
    }

}

build.gradle

apply plugin: 'java'

repositories {
    mavenCentral()
}

dependencies {
    testCompile 'junit:junit:4.11'
}

TransferContext.java

package org.kt3k.bankaccount;

public class TransferContext {

    private BankAccountSender sender;
    private BankAccountReceiver receiver;

    public TransferContext(BankAccount sender, BankAccount receiver) {
        this.sender = new BankAccountSender(sender);
        this.receiver = new BankAccountReceiver(receiver);
    }

    static private class BankAccountSender {

        private BankAccount actor;

        public BankAccountSender(BankAccount actor) {
            this.actor = actor;
        }

        public void send(Integer money, BankAccountReceiver receiver) {
            this.actor.decrease(money);

            receiver.onReceive(money);
        }
    }

    static private class BankAccountReceiver {

        private BankAccount actor;

        public BankAccountReceiver(BankAccount actor) {
            this.actor = actor;
        }

        public void onReceive(Integer money) {
            this.actor.increase(money);
        }
    }

    public void transfer(Integer money) {
        this.sender.send(money, this.receiver);
    }
    
}

TransferContextText.java

package org.kt3k.bankaccount;

import org.junit.Test;
import static org.junit.Assert.*;

public class TransferContextTest {

    @Test
    public void testTransfer() {
        BankAccount a = new BankAccount("abc123", 15000);
        BankAccount b = new BankAccount("def456", 25000);

        new TransferContext(a, b).transfer(5000);

        assertEquals((Integer)10000, a.getBalance());
        assertEquals((Integer)30000, b.getBalance());
    }

}