String --> char[]:调用String的toCharArray()

char[] --> String:调用String的构造器

    @Test
    public void test2(){
    String str1 = "abc123";  //题目： a21cb3
    char[] charArray = str1.toCharArray();

    for (int i = 0; i < charArray.length; i++) {
        System.out.println(charArray[i]);
    }

    char[] arr = new char[]{'h','e','l','l','o'};
    String str2 = new String(arr);
    System.out.println(str2);
}

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