History Manager (undo / redo)

History.java

package com.kangoapp.view.history;

import java.util.ArrayDeque;
import java.util.Deque;

public class History<ENTITY> {

    private Deque<ENTITY> backStack = new ArrayDeque<>();
    private Deque<ENTITY> forwardStack;
    private ENTITY current;
    private int max;

    public History(int max) {
        this.max = max;
    }

    public void record(ENTITY entity) {
        if (backStack.size() >= max) {
            backStack.pollFirst();
        }

        if (!entity.equals(current)) {
            current = entity;
            backStack.add(entity);
            forwardStack = new ArrayDeque<>();
        }
    }

    public ENTITY back() {
        ENTITY back = backStack.pollLast();
        if (back != null) {
            forwardStack.add(current);
        }
        if (current.equals(back)) {
            back = backStack.pollLast();
        }
        if (back != null) {
            current = back;
        }
        return back;
    }

    public ENTITY forward() {
        ENTITY forward = forwardStack.pollLast();
        if (forward != null) {
            backStack.add(current);
        }
        if (current.equals(forward)) {
            forward = forwardStack.pollLast();
        }
        if (forward != null) {
            current = forward;
        }
        return forward;
    }

    public ENTITY getCurrent() {
        return current;
    }
}

HistoryManager.java

package com.kangoapp.view.history;

public class HistoryManager {

    private final History<String> history = new History<>(1000);
    private boolean addHistoryRecord = true;

    public HistoryManager() {
        history.record("");
    }

    public History<String> getHistory() {
        return history;
    }

    public boolean isAddHistoryRecord() {
        return addHistoryRecord;
    }

    public void setAddHistoryRecord(boolean addHistoryRecord) {
        this.addHistoryRecord = addHistoryRecord;
    }

    public void enableHistoryRecording() {
        this.addHistoryRecord = true;
    }

    public void disableHistoryRecording() {
        this.addHistoryRecord = false;
    }

    public void record(String text) {
        history.record(text);
    }

    public String back() {
        return history.back();
    }

    public String forward() {
        return history.forward();
    }
}

HistoryTest.java

package com.kangoapp.view.history;

import org.junit.Assert;
import org.junit.Before;
import org.junit.Test;

public class HistoryTest {

    private History<String> history;

    @Before
    public void setUp() throws Exception {
        history = new History<>(5);
    }

    @Test
    public void testRecord() throws Exception {
        history.record("");
        history.record("a");
        history.record("b");
        history.record("c");
        history.record("d");
        Assert.assertEquals("c", history.back());
        Assert.assertEquals("b", history.back());
        Assert.assertEquals("a", history.back());
        Assert.assertEquals("", history.back());
        Assert.assertEquals(null, history.back());
        Assert.assertEquals(null, history.back());
        Assert.assertEquals("a", history.forward());
        Assert.assertEquals("b", history.forward());
        Assert.assertEquals("c", history.forward());
        Assert.assertEquals("d", history.forward());
        Assert.assertEquals(null, history.forward());
        Assert.assertEquals(null, history.forward());
        Assert.assertEquals("c", history.back());
        Assert.assertEquals("b", history.back());
        Assert.assertEquals("a", history.back());
    }

    @Test
    public void testMax() throws Exception {
        history.record("");
        history.record("a");
        history.record("b");
        history.record("c");
        history.record("d");
        history.record("e");
        history.record("f");
        Assert.assertEquals("e", history.back());
        Assert.assertEquals("d", history.back());
        Assert.assertEquals("c", history.back());
        Assert.assertEquals("b", history.back());
        Assert.assertEquals(null, history.back());
        Assert.assertEquals(null, history.back());
        Assert.assertEquals("c", history.forward());
        Assert.assertEquals("d", history.forward());
        Assert.assertEquals("e", history.forward());
        Assert.assertEquals("f", history.forward());
        Assert.assertEquals(null, history.forward());
    }
}