TreeMap uses compare/compareTo

[Douglas Boff Nandi]

Given the following code:

public class StrComparator implements Comparator<String> {
	public static void main(String[] args) {
		TreeMap<String,String> m = new TreeMap<String,String>(new StrComparator());
		m.put("a", "a");m.put("b", "b");m.put("c", "c");
		System.out.println(m);
		
		TreeMap<MyNinguem,String> m2 = new TreeMap<MyNinguem,String>();
		m2.put(new MyNinguem("a"), "a");m2.put(new MyNinguem("b"), "b");m2.put(new MyNinguem("c"), "c");
		System.out.println(m2);
		
	}

	public int compare(String o1, String o2) {
		return 0;
	}
}

class MyNinguem implements Comparable<MyNinguem> {
	private String name;
	MyNinguem (String name){
		this.name = name;
	}
	public String toString() {
		return name;
	}
	public int compareTo(MyNinguem o) {
		return 0;
	}
	public boolean equals(Object o) {
		return false;
	}
}

Why the output is
{a=c}
{a=c}
?
The code uses compareTo to define if a key already exists in the map? Can anyone explain this? Thank you all

[Henry Wong]

The code uses compareTo to define if a key already exists in the map? Can anyone explain this? Thank you all

The purpose of the compareTo() is to report if two objects are less than, greater than, or equal -- in this example, the objects are always equal. Meaning besides the first key, adding any key will report that the key is already in the map.

Now... here the relevant quote for the put() method....

put
public V put(K key, V value)
Associates the specified value with the specified key in this map. If the map previously contained a mapping for this key, the old value is replaced.

So, first, you put {a=a} into the map. Then when you try to put {b=b} into the map, it will determine that the "b" key is already in the map (because it is equal to the "a" key) and replace the value, resulting in {a=b}. Finally, the same thing happens when you try to put {c=c} into the map, resulting in {a=c}.

Henry

[Douglas Boff Nandi]

Thank you! Now its clear for me.