equals() and hashCode() contract in Java

EqHash

package com.westudio.java;

import java.util.HashMap;
import java.util.Map;

class Apple {

    public String color;

    public Apple(String color) {
        this.color = color;
    }

    @Override
    public boolean equals(Object obj) {
        if (!(obj instanceof Apple)) {
            return false;
        }

        if (obj == this) {
            return true;
        }

        return ((Apple) obj).color.equals(this.color);
    }

    @Override
    public int hashCode() {
        return this.color.length();
    }
}

public class EqHash {

    public static void main(String[] args) {
        Map<Apple, Integer> map = new HashMap<Apple, Integer>();
        map.put(new Apple("green"), 20);
        map.put(new Apple("red"), 30);
        // If not override the hashCode method this will get null
        System.out.println(map.get(new Apple("green")));
    }
}