public static void main(String[] args) {
    String s1 = "a";
    String str1="";
    String str2="";
    /**
     * 計算concat所用時間
     */
    long str1Start = System.currentTimeMillis();
    for (int i=0;i<10000;i++){
        str1=str1.concat(s1);
    }
    long str1End = System.currentTimeMillis();
    System.out.println("concat計算時間為：" + (str1End - str1Start));
    /**
     * 計算+所用時間
     */
    long str2Start = System.currentTimeMillis();
    for (int i=0;i<10000;i++){
        str2=str2+s1;
    }
    long str2End = System.currentTimeMillis();
    System.out.println("+計算時間為：" + (str2End - str2Start));
}

public static void main(String[] args) {
    // example1
    String str1 = "s1";
    System.out.println(str1 + 100);//s1100
    System.out.println(100 + str1);//100s1
 
    String str2 = "s2";
    str2 = str2.concat("a").concat("bc");
    System.out.println(str2);//s2abc
 
    // example2
    String str3 = "s3";
    System.out.println(str3 + null);//s3null
    System.out.println(null + str3);//nulls3
 
    String str4 = null;
    System.out.println(str4.concat("a"));//NullPointerException
    System.out.println("a".concat(str4));//NullPointerException
}

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);
}