Java8 StringJoiner

public StringJoiner(CharSequence delimiter) {
    // 这里只是调用了第二个构造方法，前缀和后缀传入空字符串，表示没有前后缀
    this(delimiter, "", "");
}

public StringJoiner(CharSequence delimiter,
                    CharSequence prefix,
                    CharSequence suffix) {
    // 做了以下判断，如果分隔符，前后缀为null，则抛出NullPointerException异常
    Objects.requireNonNull(prefix, "The prefix must not be null");
    Objects.requireNonNull(delimiter, "The delimiter must not be null");
    Objects.requireNonNull(suffix, "The suffix must not be null");
    
    // 赋值给当前对象的属性
    this.prefix = prefix.toString();
    this.delimiter = delimiter.toString();
    this.suffix = suffix.toString();
    this.emptyValue = this.prefix + this.suffix;
}

// 当前StringJoiner对象前缀
private final String prefix;
// 每个添加元素的分隔符
private final String delimiter;
// 当前StringJoiner对象后缀
private final String suffix;
// 前缀+元素+分隔符+后缀的值，如果没有添加元素，那么value是null
private StringBuilder value;
// 前缀+后缀的值，如果没有前后缀，那么这个值为空字符串，可以理解为value的副本，在value为null时，用它来代替
private String emptyValue;

public static void main(String[] args) {
        StringJoiner sj = new StringJoiner(",");
        String str = sj.add("a").add("b").add("c").toString();

        //a,b,c
        System.out.println(str);
    }

public static void main(String[] args) {
        List<String> list = Arrays.asList("a", "b", "c");

        String str = list.stream()
                .collect(Collectors.joining(","));

        // a,b,c
        System.out.println(str);
    }

public static void main(String[] args) {
        Cat cat1 = new Cat();
        cat1.setName("a");
        cat1.setAge(1);

        Cat cat2 = new Cat();
        cat2.setName("b");
        cat2.setAge(2);

        Cat cat3 = new Cat();
        cat3.setName("c");
        cat3.setAge(3);

        List<Cat> list = Arrays.asList(cat1, cat2, cat3);
        String str = list.stream().map(Cat::getName).collect(Collectors.joining(","));

        // a,b,c
        System.out.println(str);
    }

public static void main(String[] args) {
        StringJoiner sj = new StringJoiner(",","[", "]");
        String str = sj.add("a").add("b").add("c").toString();

        //[a,b,c]
        System.out.println(str);
    }

public static void main(String[] args) {
        StringJoiner sj1 = new StringJoiner(",");
        String str1 = sj1.add("a").add("b").add("c").toString();
        //a,b,c
        System.out.println(str1);

        StringJoiner sj2 = new StringJoiner(",");
        String str2 = sj2.add("d").add("e").add("f").toString();
        //d,e,f
        System.out.println(str2);

        String str3 = sj1.merge(sj2).toString();
        //a,b,c,d,e,f
        System.out.println(str3);
    }

public static void main(String[] args) {
        // 第一个参数是分隔符，第二个参数是list
        //打印a,b,c
        System.out.println(String.join(",", Arrays.asList("a", "b", "c")));

        // 第二个参数是可变数组
        //打印a,b,c
        System.out.println(String.join(",", "a", "b", "c"));
    }