Practice›Java Fundamentals›Module 5. Arrays & Lists›5.1 Arrays

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5.1 Arrays — default values and arrays of objects

Lesson

Что лежит в массиве до того, как вы туда что-то положили

new int[3] does not leave garbage — Java fills the slots with a default: 0 for numbers, false for booleans, and null for anything that holds an object, including String.

Осторожно с массивом объектов

Dog[] pack = new Dog[2];
System.out.println(pack[0]);   // null — НЕ собака!

This is the trap: new Dog[2] creates room for two dogs, but no dogs. Every slot is null until you build an object and put it there. Touching pack[0].name right away crashes with a NullPointerException.

Rule: creating an array of objects and filling it are two separate steps.

▶ Try it: run this, then change something and run it again.

class Dog {
    String name;
}

public class Main {
    public static void main(String[] args) {
        int[] nums = new int[3];
        String[] words = new String[3];
        Dog[] pack = new Dog[2];
        System.out.println(nums[0]);
        System.out.println(words[0]);
        System.out.println(pack[0]);
        System.out.println(pack[0] == null);
    }
}

new int[3] fills the slots with...

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new Dog[5] creates...

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Type exactly what this program prints.

public class Main { public static void main(String[] a){
    int[] n = new int[2];
    String[] s = new String[2];
    System.out.println(n[1]);
    System.out.println(s[1]);
}}

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Why does pack[0].name crash right after new Dog[2]?

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True or false?

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