String s6=new String("jiajun");
        String s1="jiajun";
        String s2="jiajun";
        System.out.println(s1==s2);//true
        System.out.println(s1==s6);//false

public String substring(int beginIndex) {
        if (beginIndex < 0) {
            throw new StringIndexOutOfBoundsException(beginIndex);
        }
        int subLen = value.length - beginIndex;
        if (subLen < 0) {
            throw new StringIndexOutOfBoundsException(subLen);
        }
        return (beginIndex == 0) ? this : new String(value, beginIndex, subLen);
    }
    
    public String concat(String str) {
        int otherLen = str.length();
        if (otherLen == 0) {
            return this;
        }
        int len = value.length;
        char buf[] = Arrays.copyOf(value, len + otherLen);
        str.getChars(buf, len);
        return new String(buf, true);
    }

public StringBuilder() {
        super(16);
    }
AbstractStringBuilder(int capacity) {
        value = new char[capacity];
    }

public AbstractStringBuilder append(String str) {
        if (str == null)
            return appendNull();
        int len = str.length();
        ensureCapacityInternal(count + len);
        str.getChars(0, len, value, count);
        count += len;
        return this;
    }
    private void ensureCapacityInternal(int minimumCapacity) {
        // overflow-conscious code
        if (minimumCapacity - value.length > 0) {
            value = Arrays.copyOf(value,
                    newCapacity(minimumCapacity));
        }
    }
    public static char[] copyOf(char[] original, int newLength) {
        char[] copy = new char[newLength];
        System.arraycopy(original, 0, copy, 0,
                         Math.min(original.length, newLength));
        return copy;
    }

public synchronized StringBuffer append(StringBuffer sb) {
        toStringCache = null;
        super.append(sb);
        return this;
    }
 public synchronized StringBuffer append(String str) {
        toStringCache = null;
        super.append(str);
        return this;
    }
    

 public StringBuilder append(StringBuffer sb) {
        super.append(sb);
        return this;
    }
    public StringBuilder append(String str) {
        super.append(str);
        return this;
    }
    public String toString() {
        // Create a copy, don‘t share the array
        return new String(value, 0, count);
    }public synchronized StringBuffer append(StringBuffer sb) {
        toStringCache = null;
        super.append(sb);
        return this;
    }
 public synchronized StringBuffer append(String str) {
        toStringCache = null;
        super.append(str);
        return this;
    }
    

 public StringBuilder append(StringBuffer sb) {
        super.append(sb);
        return this;
    }
    public StringBuilder append(String str) {
        super.append(str);
        return this;
    }
    public String toString() {
        // Create a copy, don‘t share the array
        return new String(value, 0, count);
    }

 public synchronized String toString() {
        if (toStringCache == null) {
            toStringCache = Arrays.copyOfRange(value, 0, count);
        }
        return new String(toStringCache, true);
    }
public String toString() {
        // Create a copy, don‘t share the array
        return new String(value, 0, count);
    }

public class d {
    public static void main(String[] args) {
        String s="I"+"love"+"jiajun";
        String s1="I";
        String s2="love";
        String s3="jiajun";
        String s4=s1+s2+s3;
    }
}

public class d {
    public static void main(String[] args) {
        String s="I"+"love"+"jiajun";
        String s1="I";
        String s2=s1+"lovejiajun";
        System.out.println(s==s2);
    }
}

public class d {
    public static void main(String[] args) {
        
        final String s1="I";
        
        String s2=s1+"lovejiajun";
        String s3="Ilovejiajun";
        //s1==s3
    }
}

public class Demo3 {
    public static void main(String[] args) {
        run1();
        run2();
    }
    
    public static void run1() {
        long start = System.currentTimeMillis();
        String result = "";
        for (int i = 0; i < 10000; i++) {
            result += i;
        }
        System.out.println(System.currentTimeMillis() - start);
    }

    public static void run2() {
         long start = System.currentTimeMillis();
        StringBuilder builder = new StringBuilder();
        for (int i = 0; i < 10000; i++) {
            builder.append(i);
        }
        System.out.println(System.currentTimeMillis() - start);
    }

public class Demo3 {
    public static void main(String[] args) {
        test1();
        test2();
    }

    public static void test1() {
        StringBuilder sb = new StringBuilder(7000000);
        long start = System.currentTimeMillis();
        for (int i = 0; i < 1000000; i++) {
            sb.append("jiajun");
        }
        long end=System.currentTimeMillis()-start;
        System.out.println(end);
    }

    public static void test2() {

        StringBuilder sb = new StringBuilder();
        long start = System.currentTimeMillis();
        for (int i = 0; i < 1000000; i++) {
            sb.append("jiajun");
        }
        long end=System.currentTimeMillis()-start;
        System.out.println(end);
    }
    //输出：18 26
}