Java8 新特性 String.join

先看列子：
List names=new ArrayList();

names.add(“1”);

names.add(“2”);

names.add(“3”);

System.out.println(String.join("-", names));

String[] arrStr=new String[]{“a”,“b”,“c”};

System.out.println(String.join("-", arrStr));

输出：
1-2-3
a-b-c
再看String.join()方法源码：
数组方式：

public static String join(CharSequence delimiter, CharSequence... elements) {
        Objects.requireNonNull(delimiter);
        Objects.requireNonNull(elements);
        // Number of elements not likely worth Arrays.stream overhead.
        StringJoiner joiner = new StringJoiner(delimiter);
        for (CharSequence cs: elements) {
            joiner.add(cs);
        }
        return joiner.toString();
    }

集合方式

public static String join(CharSequence delimiter,
            Iterable<? extends CharSequence> elements) {
        Objects.requireNonNull(delimiter);
        Objects.requireNonNull(elements);
        StringJoiner joiner = new StringJoiner(delimiter);
        for (CharSequence cs: elements) {
            joiner.add(cs);
        }
        return joiner.toString();
    }

StringJoiner构造器源码：

//1个参数构造器
public StringJoiner(CharSequence delimiter) {
    //调用3个参数构造器
    this(delimiter, "", "");
}
 
//3个参数构造器
//delimiter 分隔符
//prefix 前缀
//suffix 后缀
public StringJoiner(CharSequence delimiter,
                    CharSequence prefix,
                    CharSequence suffix) {
    //判断是否为null，如果为null，抛出NullPointerException
    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");
    //为成员变量赋值
    //前缀
    this.prefix = prefix.toString();
    //分隔符
    this.delimiter = delimiter.toString();
    //后缀
    this.suffix = suffix.toString();
    this.emptyValue = this.prefix + this.suffix;
}

String.join()方法中是通过add方法拼接字符串的，add()源码如下：

public StringJoiner add(CharSequence newElement) {
    //prepareBuilder()返回参数，调用append()方法
    prepareBuilder().append(newElement);
    return this;
}

perpareBuilder()方法源码如下：

private StringBuilder prepareBuilder() {
    if (value != null) {
        value.append(delimiter);
    } else {
        value = new StringBuilder().append(prefix);
    }
    return value;
}

perpareBuilder()方法返回值是一个StringBuilder对象，通过调用StringBuilder.append()方法拼接字符串。