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";
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 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。
（以上仅为个人学习总结，有不正之处敬请指正！）