String  a = “abc”；//在字串池中找abc，如果有，就直接返回地址，如果沒有就加值abc然後再返回地址（此方式的值是存放在字串池中）

String  b =  “abc”；

String  c  =   new String("abc");//在字串池中找abc，如果有，就不管，如果沒有就在物件池中加個abc，然後在堆中建一個物件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方法說明

如果是String物件就呼叫string的equals，就比較他的值

如果是其他物件，預設呼叫Object的equals，比較是不是同一個引用和==相同，

如果是其他物件，而且自己實現了equals方式，就按照他自己的equals物件的比較方式。

Object equal方法的原始碼

public boolean equals(Object obj) {

return (this == obj);// 判斷呼叫物件和傳入物件的地址是不是一樣的

}

Stirng 的 equal 方法的原始碼

public boolean equals(Object anObject) {

// 自己跟自己比

if (this == anObject) {

return true;

}

if (anObject instanceof String) {// 判斷是 String 的例項

String anotherString = (String)anObject;// 強制轉化 , 以防父類用子類的方法用不到

// 一個字元一個字元的比較

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 的字面賦值方式 




public String intern()


Returns a canonical (標準)representation for the string object. 
A pool of strings, 

initially empty, is maintained(維護) 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(決定) 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  (所有虛擬機器都必須要實現的東西)



字串池裡面是不會丟的, 而堆裡面是會被回收的,使用字面建string效率會高.



string的hashcode的原始碼


/**

     * 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]公式來生成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;

    }