原来String是这样的(下)
前言
我们讲到了String的equals和==的区别：
equals根据你编写的方法体来进行比较，而==是根据比较的引用地址是否相同来比较的。
今天我们来讲讲String类的其他.方法在源码中的解读.
publicclassTest{
publicstaticvoidmain(String[] args){
String a1=newString("abc");
String a2=newString("abc");
System.out.println(a1.isEmpty());
System.out.println(a1.length());
System.out.println(a1.charAt(1);
System.out.println(a1.substring(2,3));
}
}
这里我们介绍了String的几个简单的方法。
.isEmpty();
.length();
.charAt();
.subString();
我们查看源码就可以知道：String方法的几个构造器
.isEmpty();
/**
* Returns {@codetrue} if, and only if, {@link#length()} is {@code0}.
*
*@return{@codetrue} if {@link#length()} is {@code0}, otherwise
* {@codefalse}
*
*@since1.6
*/
publicbooleanisEmpty(){
returnvalue.length ==0;
}
isEmpty直接判断传进来的值长度是否为0
.length();
/**
* Returns the length of this string.
* The length is equal to the number of Unicode
* code units in the string.
*
*@returnthe length of the sequence of characters represented by this
* object.
*/
publicintlength(){
returnvalue.length;
}
返回字符串的长度
.charAt();
/**
* Returns the {@codechar} value at the
* specified index. An index ranges from {@code0} to
* {@codelength() - 1}. The first {@codechar} value of the sequence
* is at index {@code0}, the next at index {@code1},
* and so on, as for array indexing.
*
*
If the {@codechar} value specified by the index is a
* surrogate, the surrogate
* value is returned.
*
*@paramindex the index of the {@codechar} value.
*@returnthe {@codechar} value at the specified index of this string.
* The first {@codechar} value is at index {@code0}.
*@exceptionIndexOutOfBoundsException if the {@codeindex}
* argument is negative or not less than the length of this
* string.
*/
publiccharcharAt(intindex){
if((index <0) || (index >= value.length)) {
thrownewStringIndexOutOfBoundsException(index);
}
returnvalue[index];
}
charAt这里我们可以看到，类型为char，定义了一个下标
如果下标index<0或者>=字符的长度抛出StringIndexOutOfBoundsException
否则返回value[index] 这里我们返回的就是vlaue[1] 答案为b
.subString();
**
* Returns a string that is a substring of this string. The
* substring begins at the specified {@code beginIndex} and
* extends to the character at index {@code endIndex - 1}.
* Thus the length of the substring is {@code endIndex-beginIndex}.
*
* Examples:
*
* "hamburger".substring(4, 8) returns "urge"
* "smiles".substring(1, 5) returns "mile"
*
*
* @param beginIndex the beginning index, inclusive.
* @param endIndex the ending index, exclusive.
* @return the specified substring.
* @exception IndexOutOfBoundsException if the
* {@code beginIndex} is negative, or
* {@code endIndex} is larger than the length of
* this {@code String} object, or
* {@code beginIndex} is larger than
* {@code endIndex}.
*/
public String substring(int beginIndex, int endIndex) {
if (beginIndex < 0) {
throw new StringIndexOutOfBoundsException(beginIndex);
}
if (endIndex > value.length) {
throw new StringIndexOutOfBoundsException(endIndex);
}
int subLen = endIndex - beginIndex;
if (subLen < 0) {
throw new StringIndexOutOfBoundsException(subLen);
}
return ((beginIndex == 0) && (endIndex == value.length)) ? this
: new String(value, beginIndex, subLen);
}
subString方法有两个参数，一个起始指标，一个结束指标
判断beginIndex < 0抛出StringIndexOutOfBoundsException,同样的结束index大于长度抛出StringIndexOutOfBoundsException
然后定义一个sbuLen，为endIndex - beginIndex,判断subLen小于0抛出StringIndexOutOfBoundsException
返回如果(beginIndex == 0) && (endIndex == value.length)返回对象本身，否则该数组和beginIndex、subLen`构成新的对象返回
这里讲的就是String基本的几个方法。这里还有一个小彩蛋就是&和&&的区别
大家可以回忆一下.
&：方法无论前值是否为true,都要判断后面的逻辑表达式
&&:会形成短路，前面为false的时候就不执行后面逻辑
结尾附上String的基本方法:
/**
* Initializes a newly created {@codeString} object so that it represents
* an empty character sequence. Note that use of this constructor is
* unnecessary since Strings are immutable.
*/
publicString(){
this.value ="".value;
}
/**
* Initializes a newly created {@codeString} object so that it represents
* the same sequence of characters as the argument; in other words, the
* newly created string is a copy of the argument string. Unless an
* explicit copy of {@codeoriginal} is needed, use of this constructor is
* unnecessary since Strings are immutable.
*
*@paramoriginal
* A {@codeString}
*/
publicString(String original){
this.value = original.value;
this.hash = original.hash;
}
这就是String构成其实是一个数组，有value和hash两个属性，大家可以多多理解和想象一下.
