jdk源码系列：String为啥是不能有子类？以及内内部实现

public final class String implements java.io.Serializable,Comparaable<String>,CharSequence{
	
	private final char value[];
	
	private int hash;
	
	private static final long serialVersionUID=-6849794470754667710L;
	
	private static final ObjectStreamField[] serialPersistentField=new ObjectStreamField();

public String(){
		this.value="".value;
	}
	public String(String original){
		this.value=original.value;
		this.hash=original.hash;
	}
	
	public String(char value[]){
		this.value=Arrays.copyOf(value,value.length);
	}
	
	public String(char value[],int offset,int count){
		if(offset<0){
			throw new StringIndexOutBoundsException(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 -1>>>1
		if(offset>value.length-count){
			throw new StringIndexOutBoundsException(offset+count);
		}
		this.value=Arrays.copyOfRange(value,offset,offset+count);
	}