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);
        System.out.println(m.get("z") + " " + m.size());
    }
}

null 1

Expecting 0: a missing key gives null, not the zero of int; call getOrDefault("z", 0) when you need a number.
Expecting an exception: get returns null for a missing key; the exception comes only when that null is unboxed, as in int n = m.get("z");.

How can you tell a missing key from a key stored with the value null?
What does m.getOrDefault("z", 0) return here?

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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);
        System.out.println(m.get("z") + " " + m.size());
    }
}

null 1

Expecting 0: a missing key gives null, not the zero of int; call getOrDefault("z", 0) when you need a number.
Expecting an exception: get returns null for a missing key; the exception comes only when that null is unboxed, as in int n = m.get("z");.

How can you tell a missing key from a key stored with the value null?
What does m.getOrDefault("z", 0) return here?

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