JuniorTheoryCommon

What does the program print?

JAVA

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

public class Main {
    public static void main(String[] args) {
        Map<String, Integer> m = new HashMap<>();
        m.put("a", 1);
        m.put("a", 2);
        System.out.println(m.get("a") + " " + m.size());
    }
}

2 1

Expecting 1 1: unlike add in a HashSet, put with an existing key does not keep the old entry, it replaces the value; it even returns the old value, 1.
Expecting a size of 2: a map holds each key once, and size counts keys, so two put calls with one key leave one pair.

What does the second put return here?
How do you add a pair only if the key is not in the map yet?

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JuniorTheoryCommon

What does the program print?

JAVA

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

public class Main {
    public static void main(String[] args) {
        Map<String, Integer> m = new HashMap<>();
        m.put("a", 1);
        m.put("a", 2);
        System.out.println(m.get("a") + " " + m.size());
    }
}

2 1

Expecting 1 1: unlike add in a HashSet, put with an existing key does not keep the old entry, it replaces the value; it even returns the old value, 1.
Expecting a size of 2: a map holds each key once, and size counts keys, so two put calls with one key leave one pair.

What does the second put return here?
How do you add a pair only if the key is not in the map yet?

Back to questions Open topic in roadmap →Answer it yourself — Quiz