字符串链接+和concat的差异对比

日常工作中，字符串的链接应该是比较常见的，小编研究了一下+和concat的差异

Demo

 public static void main(String[] args) throws Exception {
        String s1 = "hello";
        String s2 = " peter";
        System.out.println(s1 + s2);
        System.out.println(s1.concat(s2));
        System.out.println(s1+112);
        System.out.println(s1.concat(1));//异常
    }

Result

hello peter
hello peter

concat源码

    /**
     * Concatenates the specified string to the end of this string.
     * <p>
     * If the length of the argument string is {@code 0}, then this
     * {@code String} object is returned. Otherwise, a
     * {@code String} object is returned that represents a character
     * sequence that is the concatenation of the character sequence
     * represented by this {@code String} object and the character
     * sequence represented by the argument string.<p>
     * Examples:
     * <blockquote><pre>
     * "cares".concat("s") returns "caress"
     * "to".concat("get").concat("her") returns "together"
     * </pre></blockquote>
     *
     * @param   str   the {@code String} that is concatenated to the end
     *                of this {@code String}.
     * @return  a string that represents the concatenation of this object's
     *          characters followed by the string argument's characters.
     */
    public String concat(String str) {
        int otherLen = str.length();
        if (otherLen == 0) {
            return this;
        }
        int len = value.length;
        char buf[] = Arrays.copyOf(value, len + otherLen);
        str.getChars(buf, len);
        return new String(buf, true);
    }

差异结果

1. +可以是字符串或者数字及其他基本类型数据，而concat只能接收字符串。

2. +左右可以为null，concat为会空指针。

3. 如果拼接空字符串，concat会稍快，在速度上两者可以忽略不计，如果拼接更多字符串建议用StringBuilder或StringBuffer 二者的区别参考StringBuffer和StringBuilder对比。

4. 从字节码来看+号编译后就是使用了StringBuiler来拼接，所以一行+++的语句就会创建一个StringBuilder，多条+++语句就会创建多个，所以为什么建议用StringBuilder的原因。