前言
我們講到了String的equals和==的區(qū)別：
equals根據(jù)你編寫的方法體來進(jìn)行比較，而==是根據(jù)比較的引用地址是否相同來比較的。
今天我們來講講String類的其他.方法在源碼中的解讀.
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));
}
}
這里我們介紹了String的幾個(gè)簡單的方法。
.isEmpty();
.length();
.charAt();
.subString();
我們查看源碼就可以知道：String方法的幾個(gè)構(gòu)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直接判斷傳進(jìn)來的值長度是否為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;
}
返回字符串的長度
.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這里我們可以看到，類型為char，定義了一個(gè)下標(biāo)
如果下標(biāo)index<0或者>=字符的長度拋出StringIndexOutOfBoundsException
否則返回value[index] 這里我們返回的就是vlaue[1] 答案為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方法有兩個(gè)參數(shù)，一個(gè)起始指標(biāo)，一個(gè)結(jié)束指標(biāo)
判斷beginIndex < 0拋出StringIndexOutOfBoundsException,同樣的結(jié)束index大于長度拋出StringIndexOutOfBoundsException
然后定義一個(gè)sbuLen，為endIndex - beginIndex,判斷subLen小于0拋出StringIndexOutOfBoundsException
返回如果(beginIndex == 0) && (endIndex == value.length)返回對(duì)象本身，否則該數(shù)組和beginIndex、subLen`構(gòu)成新的對(duì)象返回
這里講的就是String基本的幾個(gè)方法。這里還有一個(gè)小彩蛋就是&和&&的區(qū)別
大家可以回憶一下.
&：方法無論前值是否為true,都要判斷后面的邏輯表達(dá)式
&&:會(huì)形成短路，前面為false的時(shí)候就不執(zhí)行后面邏輯
結(jié)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;
}
這就是String構(gòu)成其實(shí)是一個(gè)數(shù)組，有value和hash兩個(gè)屬性，大家可以多多理解和想象一下.
