java API 有写: String 类代表字符串。Java 程序中的所有字符串字面值(如 "abc" )都作为此类的实例来实现。 字符串是常量;它们的值在创建之后不能改变。字符串缓冲区支持可变的字符串。因为 String 对象是不可变的,所以可以共享它们。例如: String str = "abc"; 等效于: char data[] = {'a', 'b', 'c'}; String str = new String(data);The String class represents character strings. All string literals in Java programs, such as "abc", are implemented as instances of this class.Strings are constant; their values cannot be changed after they are created. String buffers support mutable strings. Because String objects are immutable they can be shared. For example: String str = "abc"; is equivalent to: char data[] = {'a', 'b', 'c'}; String str = new String(data);
String a="1"; String b=a; //the address of a is the address of b
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,与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";
b = b.intern(); System.out.println(b == c); System.out.println(b == d); c = c.intern(); System.out.println(b == c); } } 以上程序的运行结果为: false false true true true false false true true false true true 从运行结果可以验证前面所述的内容。如果不懂String 类的intern()方法的用法可以参考jdk自带的文档: 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 SpecificationReturns: a string that has the same contents as this string, but is guaranteed to be from a pool of unique strings. 从CompareString类中我们也可以看出 == 与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。
因为String类太常用了,这样直接赋值,避免多次创建内容相同的String对象,节省空间,提高效率。
http://blog.csdn.net/ZangXT/archive/2009/05/19/4201979.aspx
String 类代表字符串。Java 程序中的所有字符串字面值(如 "abc" )都作为此类的实例来实现。 字符串是常量;它们的值在创建之后不能改变。字符串缓冲区支持可变的字符串。因为 String 对象是不可变的,所以可以共享它们。例如:
String str = "abc";
等效于:
char data[] = {'a', 'b', 'c'};
String str = new String(data);The String class represents character strings. All string literals in Java programs, such as "abc", are implemented as instances of this class.Strings are constant; their values cannot be changed after they are created. String buffers support mutable strings. Because String objects are immutable they can be shared. For example: String str = "abc";
is equivalent to: char data[] = {'a', 'b', 'c'};
String str = new String(data);
String b=a; //the address of a is the address of b
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);
}
}
以上程序的运行结果为:
false
false
true
true
true
false
false
true
true
false
true
true
从运行结果可以验证前面所述的内容。如果不懂String 类的intern()方法的用法可以参考jdk自带的文档:
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 SpecificationReturns:
a string that has the same contents as this string, but is guaranteed to be from a pool of unique strings. 从CompareString类中我们也可以看出 == 与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。