String用法详解(equal源码 ==和equal的解释、字面赋值和new赋值效率、重写了hashcode的方法解释)

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 truea.equal(b) trueb==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;

}