String与new String的区别

 Java运行环境有一个字符串池，由String类维护。执行语句String str="abc"时，首先查看字符串池中是否存在字符串"abc"，如果存在则直接将"abc"赋给str，如果不存在则先在字符串池中新建一个字符串"abc"，然后再将其赋给str。执行语句String str=new String("abc")时，不管字符串池中是否存在字符串"abc"，直接新建一个字符串"abc"（注意：新建的字符串"abc"不是在字符串池中），然后将其付给str。前一语句的效率高，后一语句的效率低，因为新建字符串占用内存空间。String
str = new String()创建了一个空字符串，和String str=new String("")相同。下面举个例子说明：

public class CompareString {

 public static void main(String[] args) {

  String a = new String();

  String aa = "";

  String aaa = new String("");

  String b = new String("asdf");

  String c = new String("asdf");

  String d = "asdf";

  

  System.out.println(a == aa);

  System.out.println(a == aaa);

  System.out.println(a.intern() == aa.intern());

  System.out.println(a.intern() == aaa.intern());

  System.out.println(d == "asdf");

  System.out.println(b == c);

  System.out.println(b == d);

  System.out.println(b.equals(c));

  System.out.println(b.equals(d));

  

  b = b.intern();

  System.out.println(b == c);

  System.out.println(b == d);

  c = c.intern();

  System.out.println(b == c);

 }

}

以上程式的运行结果为：

false

false

true

true

true

false

false

true

true

false

true

true

从运行结果能验证前面所述的内容。如果不懂String 类的intern()方法的用法能参考jdk自带的文件：

public String intern()

Returns a canonical representation for the string object. 

A pool of strings, initially empty, is maintained privately by the class String.

 

When the intern method is invoked, if the pool already contains a string equal to this String object as determined by the equals(Object) method, then the string from the pool is returned. Otherwise, this String object is added to the pool and a reference to
this String object is returned.

 

It follows that for any two strings s and t, s.intern() == t.intern() is true if and only if s.equals(t) is true.

 

All literal strings and string-valued constant expressions are interned. String literals are defined in §3.10.5 of the Java Language Specification 

Returns: 

a string that has the same contents as this string, but is guaranteed to be from a pool of unique strings.

从CompareString类中我们也能看出==和equals()的不同之处：即==比较的是两个对象的引用（即内存地址）是否相等，而equals()比较的是两个对象的值（即内存地址里存放的值）是否相等。当然equals()在个别类中被重写了那就例外了。