JavaSE基础知识记录(含源码分析)
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2024-03-23 14:46:46
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1.==和equals有什么区别?
先看代码
String s1 = "hello";
String s2 = "hello";
System.out.println("s1:"+s1+" s2:"+s2+" s1==s2:"+(s1 == s2));
System.out.println("s1:"+s1+" s2:"+s2+" s1.equals(s2):"+s1.equals(s2));
控制台输出
s1:hello s2:hello s1==s2:true
s1:hello s2:hello s1.equals(s2):true
我们发现好像这里的equals和==实现的功能是一样的
这时候我们再在s2下面new一个s3
String s3=new String("hello");
//再打印一次
System.out.println("s1:"+s1+" s3:"+s3+" s1==s3:"+(s1 == s3)); //false
System.out.println("s1:"+s1+" s3:"+s3+" s1.equals(s3):"+s1.equals(s3)); //true
这时候我们发现为什么字符串s1和字符串s3都是hello,但是输出结果显示s1!=s2呢?
这里究其原因就是“堆”和“常量池”的不同
Sting s1="hello"是在常量池中创建一个s1字符串对象
String s3=new String(“hello”)是在堆中创建一个s3对象
如果是在常量池中使用equals函数,则调用String.equals
如果是在堆中使用equals函数,则调用Object.equals
所以在常量池中立的String.equals只比较“内容”,但是在堆中的Object.equals里面只比较“内存”。下面我们看看源码:
String.equals:
/**
* Compares this string to the specified object. The result is {@code
* true} if and only if the argument is not {@code null} and is a {@code
* String} object that represents the same sequence of characters as this
* object.
*
* @param anObject
* The object to compare this {@code String} against
*
* @return {@code true} if the given object represents a {@code String}
* equivalent to this string, {@code false} otherwise
*
* @see #compareTo(String)
* @see #equalsIgnoreCase(String)
*/
public boolean equals(Object anObject) {
if (this == anObject) {
return true;
}
if (anObject instanceof String) {
String anotherString = (String)anObject;
int n = value.length;
if (n == anotherString.value.length) {
char v1[] = value;
char v2[] = anotherString.value;
int i = 0;
while (n-- != 0) {
if (v1[i] != v2[i])
return false;
i++;
}
return true;
}
}
return false;
}
Object.equals
/**
* Indicates whether some other object is "equal to" this one.
* <p>
* The {@code equals} method implements an equivalence relation
* on non-null object references:
* <ul>
* <li>It is <i>reflexive</i>: for any non-null reference value
* {@code x}, {@code x.equals(x)} should return
* {@code true}.
* <li>It is <i>symmetric</i>: for any non-null reference values
* {@code x} and {@code y}, {@code x.equals(y)}
* should return {@code true} if and only if
* {@code y.equals(x)} returns {@code true}.
* <li>It is <i>transitive</i>: for any non-null reference values
* {@code x}, {@code y}, and {@code z}, if
* {@code x.equals(y)} returns {@code true} and
* {@code y.equals(z)} returns {@code true}, then
* {@code x.equals(z)} should return {@code true}.
* <li>It is <i>consistent</i>: for any non-null reference values
* {@code x} and {@code y}, multiple invocations of
* {@code x.equals(y)} consistently return {@code true}
* or consistently return {@code false}, provided no
* information used in {@code equals} comparisons on the
* objects is modified.
* <li>For any non-null reference value {@code x},
* {@code x.equals(null)} should return {@code false}.
* </ul>
* <p>
* The {@code equals} method for class {@code Object} implements
* the most discriminating possible equivalence relation on objects;
* that is, for any non-null reference values {@code x} and
* {@code y}, this method returns {@code true} if and only
* if {@code x} and {@code y} refer to the same object
* ({@code x == y} has the value {@code true}).
* <p>
* Note that it is generally necessary to override the {@code hashCode}
* method whenever this method is overridden, so as to maintain the
* general contract for the {@code hashCode} method, which states
* that equal objects must have equal hash codes.
*
* @param obj the reference object with which to compare.
* @return {@code true} if this object is the same as the obj
* argument; {@code false} otherwise.
* @see #hashCode()
* @see java.util.HashMap
*/
public boolean equals(Object obj) {
return (this == obj);
}
根据上面源码我们很容易得出来Object.equals里面的equals其实就是==写出来的。但是String.equals首先判断内存相不相等,如果相等,直接返回true,如果内存不等,就开始判断两个字符串是否内容相等。
2.最近新学Python和重学javaSE,发现其实很多Python里面好用的函数,JavaSE里面也有
比如一些切割函数:
我就直接放作者链接了:
https://www.liaoxuefeng.com/wiki/1252599548343744/1260469698963456
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