Object with composed key

ComposedKeyObject.java

public class MyObject {
    private MyObjectKey key;
    private String someAttribute;

    public MyObject(int firstKey, int secondKey, String someAttribute) {
        this.key = new MyObjectKey(firstKey, secondKey);
        this.someAttribute = someAttribute;
    }

    public MyObjectKey getKey() { return key; }

    public String getSomeAttribute() { return someAttribute; }

    public int hashCode() { return key.hashCode(); }

    public boolean equals(Object obj) { ... }
}

public class MyObjectKey {
    private int firstKey;
    private int secondKey;

    public MyObjectKey(int firstKey, int secondKey) {
        this.firstKey = firstKey;
        this.secondKey = secondKey;
    }

    public int getFirstKey() { return firstKey; }

    public int getSecondKey() { return secondKey; }

    public int hashCode() {
        int result = 31 + firstKey;
        return 31 * result + secondKey;
    }

    public boolean equals(Object obj) { ... }
}