concat method of the String class in Java

Before looking at concat (), you first need to be clear that two of the special nature of String.

• Length immutable
• Immutable values 
  of these two points of String statement can be reflected from the source code:

  private final char[] value ;

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Wherein the characteristic corresponds unchangeable final value; and char [] String corresponding to the characteristic length is unchangeable.

Therefore, when we splicing String, it should produce a new string. 
For this, we can be the source of interpretation of the concat () reached the same conclusion.

@param  str 需要拼接到原字符串的新串
@param  otherlen 新串的长度
@param  len 原字符串的长度
@param  buf 存放最终字符串的字符数组（长度为len+otherlen）
@method copyOf（char[] original, int newLength） 复制指定的数组，截取或用 null 字符填充（如有必要），以使副本具有指定的长度。 @method getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin) 将字符从此字符串复制到目标字符数组。 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); }

Source analysis:

concat（）方法首先获取拼接字符串的长度，判断这个字符串长度是否为0（判断这个用来拼接的字符串是不是空串），如果是就返回原来的字符串（等于没有拼接）；否则就获取源字符串的长度，创建一个新的char[]字符数组，这个字符数组的长度是拼接字符串的长度与源字符串的长度之和，通过Arrays类的copyOf方法复制源数组，然后通过getChars方法将拼接字符串拼接到源字符串中，然后将新串返回。

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API also this method is explained:

• If the string length parameter is 0, this String object is returned.
• Otherwise, create a new String object that represents the sequence of characters and character sequences character sequence parameter string String object represents thus represented connected together.

In summary, when a String object stitching, creates a new string to store the new string.