Java Generic Immutable Collection

Context

In object-oriented and functional programming, an immutable object is an object whose state cannot be modified after it is created. This is in contrast to a mutable object which can be modified after it is created.

Classes should be immutable unless there's a very good reason to make them mutable. If a class cannot be made immutable, limit its mutability as much as possible. The JDK contains many immutable classes, including String, the boxed primitive classes, and BigInteger and etc. Basically the immutable classes are less prone to error.

ImmutableStack

• Supports methods:

package com.cozy.collection.stack;

public interface Stack<T> {

    public Stack<T> push(T t);

    public Stack<T> pop() throws Exception;

    public T head() throws Exception;

    public boolean isEmpty();
}

• Test by using ImmutableStack:

package com.cozy.collection.stack;

import org.hamcrest.core.Is;
import org.junit.Test;

import static org.junit.Assert.*;

public class ImmutableStackTest {

    @Test
    public void createNewEmptyStack() {
        Stack emptyStack = ImmutableStack.initEmptyStack();
        assertTrue(emptyStack.isEmpty());
    }

    @Test
    public void createNewImmutableStack() throws Exception {
        Stack emptyStack = ImmutableStack.initEmptyStack();
        Stack immutableStack = emptyStack.push(5);
        immutableStack = immutableStack.push(2);

        assertThat(immutableStack.head(), Is.is(2));
        immutableStack = immutableStack.pop();
        assertThat(immutableStack.head(), Is.is(5));
        immutableStack = immutableStack.pop();
        assertTrue(immutableStack.isEmpty());
    }
}

ImmutableQueue

• Supports methods:

package com.cozy.collection.queue;

public interface Queue<T> {

    public Queue<T> enQueue(T t);

    public Queue<T> deQueue() throws Exception;

    public T head() throws Exception;

    public boolean isEmpty();
}

• Test by using ImmutableQueue:

package com.cozy.collection.queue;

import org.hamcrest.core.Is;
import org.junit.Test;

import static org.junit.Assert.*;

public class ImmutableQueueTest {

    @Test
    public void createNewEmptyQueue() {
        Queue emptyQueue = ImmutableQueue.getEmptyQueue();
        assertTrue(emptyQueue.isEmpty());
    }

    @Test
    public void createNewImmutableQueue() throws Exception {
        Queue emptyQueue = ImmutableQueue.getEmptyQueue();
        Queue immutableQueue = emptyQueue.enQueue(5);
        immutableQueue = immutableQueue.enQueue(2);

        assertThat(immutableQueue.head(), Is.is(5));
        immutableQueue = immutableQueue.deQueue();
        assertThat(immutableQueue.head(), Is.is(2));
        immutableQueue = immutableQueue.deQueue();
        assertTrue(immutableQueue.isEmpty());
    }
}