When does StringBuffer adds strings to the String

问题:

When I define a StringBuffer variable with new, this string is not added to the String pool, right?

Now, when I define another StringBuffer but not with new, I define it as StrPrev.append("XXX") suddenly it is.(or so says my college teacher). Why is that? What makes this string to suddenly become a string-pool string?

回答1:

When I define a StringBuffer variable with new, this string is not added to the String pool, right?

Creating a StringBuffer does not create a String at all.

Now, when I define another StringBuffer but not with new, I define it as StrPrev.append("XXX") suddenly it is.

This is totally confused:

• When you call strBuff.append("XXX") you are NOT defining a new StringBuffer. You are updating the existing StringBuffer that strBuff refers to. Specifically, you are adding extra characters to the end of the buffer.

• You only get a new String from the StringBuffer when you call strBuff.toString().

• You only add a String to the string pool when you call intern() on the String. And that only adds the string to the pool if there is not already an equal string in the pool.

• The String object that represents the literal "XXX" is a member of the string pool. But that happens (i.e. the String is added to the pool) when the class is loaded, not when you execute the append call.

(If you teacher told you that StringBuffer puts strings into the Java string pool, he / she is wrong. But, given your rather garbled description, I suspect that you actually misheard or misunderstood what your teacher really said.)

回答2:

"XXX" in StrPrev.append("XXX") is a string literal that is interned at class loading time (class loading time of the class that contains the code).

"XXX" is not added to the pool by the StringBuffer.

From the JLS section 3.10.5:

Moreover, a string literal always refers to the same instance of class String. This is because string literals - or, more generally, strings that are the values of constant expressions (§15.28) - are "interned" so as to share unique instances

From the JLS section 12.5:

Loading of a class or interface that contains a String literal (§3.10.5) may create a new String object to represent that literal. (This might not occur if the same String has previously been interned (§3.10.5).)

回答3:

buf.append("XXX") followed by buf.toString(), and then returning the string to the pool. With the pool in place, only one StringBuffer object is ever allocated.

回答4:

Actually your teacher is referring to XXX . which goes to StringPool because all string literals written in java program goes to StringPool while execution...