String类的核心方法：
private final char[] value; // final修饰的char数组;
private int hashCode;
// trim方法
public String trim(){
int start = 0;
int end = this.value.length;
char[] val=this.value;
while( (start<end) && (val[start] <= ' ' )){
start++;
}
while( start<end) && (val[end] <= ' '){
end--;
}
return (start>0 || end<this.value.length) : subString(start,end) : this;
}
// subString方法
public String subString(start,end){
if(start<0){
throw new StringIndexOutOfBoundsException(start);
}
if(end>this.value.length){
throw new StringIndexOutOfBoundsException(end);
}
int sub=end-start;
if(sub<0){
throw new StringIndexOutOfBoundsException(sub);
}
return (start==0)&&(end==this.value.length) ? this : new String(value,start,sub);
}
// startsWith方法
public boolean startsWith(String prefix,int offset){
char[] source=this.value;
int sl=this.value.length;
char[] pre=prefix.value;
int pl=prefix.value.length;
if(offset<0 || offset>sl-pl){
return false;
}
int index=0;
while(--pl>=0){
if(surce[offset++]!=pre[index++]){
return false;
}
}
return true;
}
