//由于是final，所以string是不可继承的
public final class String
//final修饰的char[]代表了被存储的数据不可更改性
//是final和private
private final char value[];
//下面这个例子说明，是final和private一起来使得String的数据是不可改变的
final  int[] array={1,2,3,4};
array[2]=100;
System.out.println(array[2]);

    public String(char value[], int offset, int count) {
    if (offset < 0) {
        throw new StringIndexOutOfBoundsException(offset);
    }
    if (count <= 0) {
        if (count < 0) {
            throw new StringIndexOutOfBoundsException(count);
        }
        if (offset <= value.length) {
            this.value = "".value;
            return;
        }
    }
    // Note: offset or count might be near -1>>>1.
    if (offset > value.length - count) {
        throw new StringIndexOutOfBoundsException(offset + count);
    }
    //就是在这里将char数组拷贝过来，截取的offset和截取字符串的值是一样的
    this.value = Arrays.copyOfRange(value, offset, offset+count);
}

public String replaceFirst(String regex, String replacement) {
    return Pattern.compile(regex).matcher(this).replaceFirst(replacement);
}
    public String replaceAll(String regex, String replacement) {
    return Pattern.compile(regex).matcher(this).replaceAll(replacement);
} 
    public String replace(CharSequence target, CharSequence replacement) {
    return Pattern.compile(target.toString(), Pattern.LITERAL).matcher(
            this).replaceAll(Matcher.quoteReplacement(replacement.toString()));
} 

public StringBuilder() {
    super(16);
}

public StringBuilder(int capacity) {
    super(capacity);
}
 public StringBuilder(String str) {
    super(str.length() + 16);
    append(str);
}
    @Override
public StringBuilder append(Object obj) {
    return append(String.valueOf(obj));
}

@Override
public StringBuilder append(String str) {
    super.append(str);
    return this;
} 

@Override
public synchronized int length() {
    return count;
}

@Override
public synchronized int capacity() {
    return value.length;
}


@Override
public synchronized void ensureCapacity(int minimumCapacity) {
    super.ensureCapacity(minimumCapacity);
}

package com.test;

public class demo {
    public static void main(String[] args) {
        String a = "1";
        String b = "2";
        System.out.println(a+b);
    }
}

    //stirng不能为null，为null会报NullPointerException
    public String toString() {
    return this;
}
//这个方法的功能就是如果形参是null，那么返回null字符串，而不是直接报NullPointerException异常
public static String valueOf(Object obj) {
    return (obj == null) ? "null" : obj.toString();
}

public static String toString(int i, int radix) {
    if (radix < Character.MIN_RADIX || radix > Character.MAX_RADIX)
        radix = 10;

    /* Use the faster version */
    if (radix == 10) {
        return toString(i);
    }

    char buf[] = new char[33];
    boolean negative = (i < 0);
    int charPos = 32;

    if (!negative) {
        i = -i;
    }

    while (i <= -radix) {
        buf[charPos--] = digits[-(i % radix)];
        i = i / radix;
    }
    buf[charPos] = digits[-i];

    if (negative) {
        buf[--charPos] = '-';
    }

    return new String(buf, charPos, (33 - charPos));
}

package com.test;



public class demo {
    public static void main(String[] args) {
        String str = "world";
        switch (str) {
            case "hello":
                System.out.println("hello");
                break;
            case "world":
                System.out.println("world");
                break;
            default:
                break;
        }

    }
}

package com.test;

public class demo02 {
    public static void main(String[] args) {
        String s="1";
        String s1=new String("1");
        String s2=s.intern();
        System.out.println(s==s1);
        System.out.println(s1==s2);
        System.out.println(s2==s);
    }
}