How to use put() in a subclass of Java's TreeMap

Question

I want to create a subclass of class of java.util.TreeMap, to allow me to add an increment method:

public class CustomTreeMap<K, V> extends TreeMap<K, V> {
  public void increment(Integer key) {
    Integer intValue;

    if (this.containsKey(key)) {
      Object value = this.get(key);
      if (!(value instanceof Integer)) {
        // Fail gracefully
        return;

      } else {
        intValue = (Integer) value;
        intValue++;
      }
    } else {
      intValue = 1;
    }

    this.put(key, intValue); // put(Integer, Integer) cannot be applied in TreeMap
  }
}

Android Studio 1.0.2 first proposes put(K Key, V Value) for autocompletion, and later warns that:

put(K, V) cannot be applied in TreeMap to (java.lang.integer, java.lang.integer)

What is it that I am doing wrong?

See here for the solution I adopted.

You define both K and V as generic, why are you defining them when both are actually `Integer`? — Thomas Jungblut, Jan 08 '15 at 15:57

Zoltán · Accepted Answer · 2015-01-08T16:04:26.553

If you want to create your custom treemap to handle Integers exclusively, you should make it extend TreeMap<K, Integer>, not the generic type V:

public class CustomTreeMap<K> extends TreeMap<K, Integer> {
  ...
}

This way you don't need the instanceof check later.

If your key also needs to be an Integer, declare no generic types instead:

public class CustomTreeMap extends TreeMap<Integer, Integer> {
  ...
}

Rene M. · Answer 2 · 2015-01-08T16:15:37.127

If it should be Integer then use Integer:

public class CustomTreeMap<K> extends TreeMap<K, Integer> {
  public void increment(K key) {
    Integer intValue;

    if (this.containsKey(key)) {
      Object value = this.get(key);
      if (!(value instanceof Integer)) {
        // Fail gracefully
        return;

      } else {
        intValue = (Integer) value;
        intValue++;
      }
    } else {
      intValue = 1;
    }

    this.put(key, intValue); // put(Integer, Integer) cannot be applied in TreeMap
  }
}

How to use put() in a subclass of Java's TreeMap

2 Answers2