java.lang(String)

public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence {

    /** The value is used for character storage. */
    private final char value[];

    private int hash; // Default to 0

    public String() { //无参构造器
        this.value = new char[0];
    }

    public String(String original) {
        this.value = original.value;
        this.hash = original.hash;
    }

/*****传入一个字符数组的构造函数,使用java.utils包中的Arrays类复制******/
    public String(char value[]) {
        this.value = Arrays.copyOf(value, value.length);
    }

/*******传入一个字符串数字，和开始元素，元素个数的构造函数******/
    public String(char value[], int offset, int count) {
        if (offset < 0) {
            throw new StringIndexOutOfBoundsException(offset);
        }
        if (count < 0) {
            throw new StringIndexOutOfBoundsException(count);
        }
        // Note: offset or count might be near -1>>>1.
        if (offset > value.length - count) {
            throw new StringIndexOutOfBoundsException(offset + count);
        }
        this.value = Arrays.copyOfRange(value, offset, offset+count);
    }
/*********************类似方法不介绍了**************************/

    public String(StringBuffer buffer) {
        synchronized(buffer) {
            this.value = Arrays.copyOf(buffer.getValue(), buffer.length());
        }
    }

    public boolean equals(Object anObject) {
        if (this == anObject) {
            return true;
        }
        if (anObject instanceof String) {
            String anotherString = (String) anObject;
            int n = value.length;
            if (n == anotherString.value.length) {
                char v1[] = value;
                char v2[] = anotherString.value;
                int i = 0;
                while (n-- != 0) {
                    if (v1[i] != v2[i])
                            return false;
                    i++;
                }
                return true;
            }
        }
        return false;
    }

/***********s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]*********/
    public int hashCode() {
        int h = hash;
/***如果hash没有被计算过，并且字符串不为空，则进行hashCode计算*****/
        if (h == 0 && value.length > 0) {
            char val[] = value;

            for (int i = 0; i < value.length; i++) {
                h = 31 * h + val[i];
            }
            hash = h;
        }
        return h;
    }    

String a = "abc";
String b = new String("ab1").intern();
if ( a == b ) {
  System.out.println("a == b");  
} else {
  System.out.println("a不等于b");
}
打印出：a == b

String s="abc";
s="def";