String a = “abc”；//在字符串池中找abc，如果有，就直接返回地址，如果没有就加值abc然后再返回地址（此方式的值是存放在字符串池中）
String b = “abc”；
String c = new String("abc");//在字符串池中找abc，如果有，就不管，如果没有就在对象池中加个abc，然后在堆中建一个对象abc，返回地址（此方式值存放在堆中的）
String d = new String("abc");
a ==b true
a.equal(b) true
b==c false;
b.equal(c) true;
c==d false;
c.equal(d) true;
equals方法说明
如果是String对象就调用string的equals，就比较他的值
如果是其他对象，默认调用Object的equals，比较是不是同一个引用和==相同，
如果是其他对象，而且自己实现了equals方式，就按照他自己的equals对象的比较方式。
Object equal方法的源代码
public boolean equals(Object obj) {
return (this == obj);// 判断调用对象和传入对象的地址是不是一样的
}
Stirng 的 equal 方法的源代码
public boolean equals(Object anObject) {
// 自己跟自己比
if (this == anObject) {
return true;
}
if (anObject instanceof String) {// 判断是 String 的实例
String anotherString = (String)anObject;// 强制转化 , 以防父类用子类的方法用不到
// 一个字符一个字符的比较
int n = count;
if (n == anotherString.count) {
char v1[] = value;
char v2[] = anotherString.value;
int i = offset;
int j = anotherString.offset;
while (n-- != 0) {
if (v1[i++] != v2[j++])
return false;
}
return true;
}
}
return false;
}
java String 的字面赋值方式 public String intern() Returns a canonical (标准)representation for the string object. A pool of strings, initially empty, is maintained(维护) 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(决定) 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(字面) 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. Lexical Structure >Literals >String Literals The Java Language Specification (所有虚拟机都必须要实现的东西) 字符串池里面是不会丢的, 而堆里面是会被回收的,使用字面建string效率会高. string的hashcode的源码 /** * Returns a hash code for this string. The hash code for a * <code>String</code> object is computed as * <blockquote><pre> * s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1] * </pre></blockquote> * using <code>int</code> arithmetic, where <code>s[i]</code> is the * <i>i</i>th character of the string, <code>n</code> is the length of * the string, and <code>^</code> indicates exponentiation. * (The hash value of the empty string is zero.) * * @return a hash code value for this object. */ //就是按s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]公式来生成int序列 public int hashCode() { int h = hash; if (h == 0) { int off = offset; char val[] = value; int len = count; for (int i = 0; i < len; i++) { h = 31*h + val[off++]; } hash = h; } return h; }
