String aa = "今天";
        String bb = "明天";
        System.out.println(aa+bb);

        String aa = "今天";
        String bb = "明天";
        StringBuilder sber = new StringBuilder();
        StringBuffer sbf = new StringBuffer();
        sber.append(aa).append(bb);
        System.out.println(sber.toString());
        sbf.append(aa).append(bb);
        System.out.println(sbf.toString());

        String aa = "今天";
        String bb = "明天";
        String concat = aa.concat(bb);
        System.out.println(concat);

		String aa = "今天";
        String bb = "明天";
        String join = String.join("-", aa, bb);
        System.out.println(join);

		String aa = "今天";
        String bb = "明天";
        StringJoiner sj = new StringJoiner(":", "[", "]");
        sj.add(aa).add(bb);
        System.out.println(sj.toString());

package java.util;

public final class StringJoiner {
    private final String prefix;//前綴
    private final String delimiter;//間隔符
    private final String suffix;//后綴
    private StringBuilder value;//值

    private String emptyValue;//空值

    public StringJoiner(CharSequence delimiter) {
        this(delimiter, "", "");//默認(rèn)前綴和后綴為"",重載調(diào)用
    }

    public StringJoiner(CharSequence delimiter,
                        CharSequence prefix,
                        CharSequence suffix) {
        //間隔符，前綴和后綴判斷是否為null，null將拋出異常
        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"); 
        // 成員變量賦值
        this.prefix = prefix.toString();
        this.delimiter = delimiter.toString();
        this.suffix = suffix.toString();
        this.emptyValue = this.prefix + this.suffix;//空值被設(shè)置為只有前后綴
    }
	//設(shè)置空值，檢查是否為null
    public StringJoiner setEmptyValue(CharSequence emptyValue) {
        this.emptyValue = Objects.requireNonNull(emptyValue,
            "The empty value must not be null").toString();
        return this;
    }

    @Override
    public String toString() {
        if (value == null) {
            return emptyValue;//沒有值將返回空值或者后續(xù)設(shè)置的空值
        } else {
            if (suffix.equals("")) {
                return value.toString();//后綴為""直接返回字符串，不用添加
            } else {
	            //后綴不為""，添加后綴，然后直接返回字符串，修改長(zhǎng)度
                int initialLength = value.length();
                String result = value.append(suffix).toString();
                // reset value to pre-append initialLength
                value.setLength(initialLength);
                return result;
            }
        }
    }
    初始化，先添加前綴，有了之后每次先添加間隔符，StringBuilder后續(xù)append字符串
    public StringJoiner add(CharSequence newElement) {
        prepareBuilder().append(newElement);
        return this;
    }
	//合并StringJoiner，注意后面StringJoiner 的前綴就不要了，后面的appen進(jìn)來(lái)
    public StringJoiner merge(StringJoiner other) {
        Objects.requireNonNull(other);
        if (other.value != null) {
            final int length = other.value.length();
            // lock the length so that we can seize the data to be appended
            // before initiate copying to avoid interference, especially when
            // merge 'this'
            StringBuilder builder = prepareBuilder();
            builder.append(other.value, other.prefix.length(), length);
        }
        return this;
    }
	//初始化，先添加前綴，有了之后每次先添加間隔符
    private StringBuilder prepareBuilder() {
        if (value != null) {
            value.append(delimiter);
        } else {
            value = new StringBuilder().append(prefix);
        }
        return value;
    }

    public int length() {
        // Remember that we never actually append the suffix unless we return
        // the full (present) value or some sub-string or length of it, so that
        // we can add on more if we need to.
        //不忘添加后綴的長(zhǎng)度
        return (value != null ? value.length() + suffix.length() :
                emptyValue.length());
    }
}

		String aa = "今天";
        String bb = "明天";
        String ids[] = {"1","2","3"};
        System.out.println(StringUtils.join(aa,bb,"-","124"));
        String join1 = StringUtils.join(ids,",");
        System.out.println(join1);