ysheng wrote:
但下面这样用要减少一半的时间，虽然代码增加了1行
sbf.append("a line");
sbf.append("\r\n");

why wrote:
sbf.append("a line").append("\r\n");

ysheng wrote:
使用 StringBuffer 也需要注意效率

很多时候我们习惯这样用
sbf.append("a line"+"\r\n");

但下面这样用要减少一半的时间，虽然代码增加了1行
sbf.append("a line");
sbf.append("\r\n");

zzw109433 wrote:
String str=null;
for(int i=0;i<10;i++){
str=new String();
}
不要写成
for(int i=0;i<10;i++){
String str=new String();
}

zcjl wrote:
这个似乎搞反了吧？
倒是后一种方法是推荐的，因为每次循环生成的对象的可达域都在循环体内，循环结束即可被gc回收
而前一种写法，最后一次循环生成的对象被循环体外的reference引用着，无法即时被回收

ysheng wrote:
http://peterbull.mblogger.cn/
在进行常量与变量比较时，建议把常量放在前面（如　if (null==name){}）
eg：!name.equals("") 如果name为空的时候会造成NullPointerException
建议采用!"".equals(name)

scarecrow wrote:
>> 为每次循环生成的对象的可达域都在循环体内，循环结束即可被gc回收
你不认为这样会增加gc 的负担么？ 另外每次都要再循环体内创建一个新的String引用 str 也增加了系统的开销。

ftang wrote:
you didn't get the point here:
"a" + "b" + "c" is bad expierence because in memory JVM will create

"a", "b", "ab","c", "abc" which here, "ab" is a waste...so you should always use StringBuffer.

Get the point...know why...ALL the tips here are Computer Sciense 101, but you guys should know why....WHY!! not only know HOW

From SLS at http://java.sun.com/docs/books/jls/

15.18.1.2 Optimization of String Concatenation
An implementation may choose to perform conversion and concatenation in one step to avoid creating and then discarding an intermediate String object. To increase the performance of repeated string concatenation, a Java compiler may use the StringBuffer class or a similar technique to reduce the number of intermediate String objects that are created by evaluation of an expression.
For primitive types, an implementation may also optimize away the creation of a wrapper object by converting directly from a primitive type to a string.

zcjl wrote:
首先，这不会增加gc的负担
其次，这样做，是基于《Effective Java》里面的建议：第29条：将局部变量的作用域最小化

zcjl wrote:
首先，这不会增加gc的负担
其次，这样做，是基于《Effective Java》里面的建议：第29条：将局部变量的作用域最小化

gongshi wrote:
What you said is not necessary true, actually, not true.

ftang wrote:
true or not I think you should point out from when...JVM is improving every day... this kind of stupid problem Sun won't miss. This theory is not from my idea it is from Joshua Bloch's Effective Java Programming Language. I forgot the which page...but you can easily find it out...

plus...I bring this up here is because I want the beginner know when they learn the tricks they need to know why...

scarecrow wrote:
>>首先，这不会增加gc的负担
能给出你得出这个结论的理由么？
>>其次，这样做，是基于《Effective Java》里面的建议：第29条：将局部变量的作用域最小化

将局部变量的作用域最小化,没错。不过这应该对于if ,try switch等语句说的吧，对与循环我认为还是不妥当的，特别是当循环的次数比较大的时候。