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How to keep a 1 to 1 relation consistent

Finally, here's your solution:

public class A {
    private B b;

    public void setB(B b) {
        if(this.b != null) {
        this.b = b;
        if(b != null && b.getA() != this)

    public void unsetB() {
        this.b = null;

    public B getB() {
        return b;


public class B {
    private A a;

    public void setA(A a) {
        if(this.a != null) {
        this.a = a;
        if(a != null && a.getB() != this)

    public void unsetA() {
        this.a = null;

    public A getA() {
        return a;


My Test Class:

public class Test {

    public static void main(String ... args) {
        A a1 = new A();
        B b1 = new B();
        A a2 = new A();
        B b2 = new B();

        checkForZeroOrTwoRelations(a1, a2, b1,
        checkForZeroOrTwoRelations(a1, a2, b1,
        checkForZeroOrTwoRelations(a1, a2, b1,
        checkForZeroOrTwoRelations(a1, a2, b1,
        checkForZeroOrTwoRelations(a1, a2, b1,

    private static void
checkForZeroOrTwoRelations(A a1, A a2, B b1, B b2)
        int i = 0;
        if(a1.getB() != null) i++;
        if(a2.getB() != null) i++;
        if(b1.getA() != null) i++;
        if(b2.getA() != null) i++;
        if(i != 0 && i != 2) {
            throw new IllegalStateException();

Categories : Java

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