Thinking about the transformation between string and char[]

If it changes char[] Array ,String It doesn't change

public class Main {
    public static void main(String[] args) {
        char[] cs = "Hello".toCharArray();
        String s = new String(cs);
        System.out.println(s);
        cs[0] = 'X';
        System.out.println(s);
    }
}

Output ：

Hello
Hello 

That's because through new String(char[]) Create a new String When an instance , It doesn't directly refer to the incoming char[] Array , It's a copy , therefore , Modify the external char[] Arrays do not affect String Inside the instance char[] Array , Because these are two different arrays .

from String The invariance design can be seen , If the incoming object is likely to change , We Need copy instead of direct reference .

for example , The following code designs a Score Class saves the grades of a group of students ：

// int[]
import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        int[] scores = new int[] { 88, 77, 51, 66 };
        Score s = new Score(scores);
        s.printScores();
        scores[2] = 99;
        s.printScores();
    }
}

class Score {
    private int[] scores;
    public Score(int[] scores) {
        this.scores = scores;
    }

    public void printScores() {
        System.out.println(Arrays.toString(scores));
    }
}

Output ：

[88, 77, 51, 66]
[88, 77, 99, 66] 

Observe the output twice , because Score The internal directly refers to the external incoming int[] Array , This will cause external code to int[] Array modification , Affect the Score Class field . If the external code is not trusted , This will cause security risks .

Please repair Score Construction method of , So that the modification of the array by external code does not affect Score Example of int[] Field .

// int[]
import java.util.Arrays;

public class Main {
    public static void main(String[] args) {
        int[] scores = new int[] { 88, 77, 51, 66 };
        Score s = new Score(scores);
        s.printScores();
        scores[2] = 99;
        s.printScores();
    }
}

class Score {
    private int[] scoreAll;

    public Score(int[] scoreAll) {
        int[] ints = new int[scoreAll.length];
        for (int key = 0; key < scoreAll.length; key++) {
            ints[key] = scoreAll[key];
        }
        System.out.println(ints);
        this.scoreAll = ints;
    }

    public void printScores() {
        System.out.println(Arrays.toString(scoreAll));
    }
}

Output ：

[88, 77, 51, 66]
[88, 77, 51, 66]