AV2 CombinationLock

package AV2;

public class CombinationLock {
    private int combination;
    private boolean isOpen;
    private static int DEFAULT_COMBO = 100;

    public CombinationLock(int combination) {
        if (isComboValid(combination)) {
            this.combination = combination;
        } else {
            this.combination = DEFAULT_COMBO;
        }
        this.isOpen = false;
    }

    private boolean isComboValid(int combination) {
        if (combination >= 100 && combination <= 999) {
            return true;
        } else
            return false;
    }

    public boolean open(int combination) {
        this.isOpen = (this.combination == combination);
        return this.isOpen;
//        if (this.combination == combination) {
//            this.isOpen = true;
//            return true;
//        } else
//            return false;
    }

    public boolean changeCombo(int oldCombo, int newCombo) {
        if ((open(oldCombo) == true) && (isComboValid(newCombo))) {
            this.combination = newCombo;
            return true;
        } else
            return false;
    }

    public boolean isOpen() {
        return isOpen;
    }

    public void lock() {
        this.isOpen = false;
    }

    public static void main(String[] args) {
        CombinationLock validLock = new CombinationLock(234);

        System.out.println("TEST IS OPEN");
        System.out.println(validLock.isOpen());

        System.out.println("TEST OPEN");
        System.out.println(validLock.open(233));
        System.out.println(validLock.open(236));
        System.out.println(validLock.open(234));

        validLock.lock();

        System.out.println("TEST CHANGE COMBINATION");
        System.out.println(validLock.changeCombo(234, 777));
        System.out.println(validLock.open(777));

        CombinationLock invalidLock = new CombinationLock(234567);

        System.out.println("TEST IS OPEN");
        System.out.println(invalidLock.isOpen());

        System.out.println("TEST OPEN");
        System.out.println(invalidLock.open(103));
        System.out.println(invalidLock.open(236));
        System.out.println(invalidLock.open(100));

        invalidLock.lock();

        System.out.println("TEST CHANGE COMBINATION");
        System.out.println(invalidLock.changeCombo(100, 900));
        System.out.println(invalidLock.open(900));
    }
}