【java基础】Java常用类———包装类

阳光穿透心脏的1/2处 2024-03-26 11:05 52阅读 0赞

### 包装类 ###

wrapper

#### 装箱与拆箱 ####

*  装箱：基本类型->包装类； 拆箱： 包装类->基本类型

public class Integer01 {
        
     	    public static void main(String[] args) {
        
     	        //演示int <--> Integer 的装箱和拆箱
     	        //jdk5前是手动装箱和拆箱
     	        //手动装箱 int->Integer
     	        int n1 = 100;
     	        Integer integer = new Integer(n1);
     	        Integer integer1 = Integer.valueOf(n1);
     	        //手动拆箱
     	        //Integer -> int
     	        int i = integer.intValue();
     	        //jdk5后，就可以自动装箱和自动拆箱
     	        int n2 = 200;
     	        //自动装箱 int->Integer
     	        Integer integer2 = n2; //底层使用的是 Integer.valueOf(n2)
     	        //自动拆箱 Integer->int
     	        int n3 = integer2; //底层仍然使用的是 intValue()方法
     	    }
     	}

如果频繁拆装箱的话，也会严重影响系统的性能。我们应该尽量避免不必要的拆装箱操作。

#### 8种包装类 ####

针对8种基本数据类型相应的引用类型——包装类

<table> 
 <thead> 
  <tr> 
   <th>基本数据类型</th> 
   <th>包装类</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>byte</td> 
   <td>Byte</td> 
  </tr> 
  <tr> 
   <td>short</td> 
   <td>Short</td> 
  </tr> 
  <tr> 
   <td>int</td> 
   <td>Integer</td> 
  </tr> 
  <tr> 
   <td>long</td> 
   <td>Long</td> 
  </tr> 
  <tr> 
   <td>char</td> 
   <td>Character</td> 
  </tr> 
  <tr> 
   <td>float</td> 
   <td>Float</td> 
  </tr> 
  <tr> 
   <td>double</td> 
   <td>Double</td> 
  </tr> 
  <tr> 
   <td>boolean</td> 
   <td>Boolean</td> 
  </tr> 
 </tbody> 
</table>

所有包装类都是抽象类Number的子类

##### 缓存机制 #####

Java 基本数据类型的包装类型的大部分都用到了缓存机制来提升性能。

`Byte`,`Short`,`Integer`,`Long` 这 4 种包装类默认创建了数值 **\[-128，127\]** 的相应类型的缓存数据，

> 只有自动装箱的才有缓存

`Character` 创建了数值在 **\[0,127\]** 范围的缓存数据，  
`Boolean` 直接返回 `True` or `False`。  
两种浮点数类型的包装类 `Float`,`Double` 并没有实现缓存机制。

Integer a1=new Integer(1);  
    Integer a2=new Integer(1);  
    System.out.println(a1==a2);//false  
      
    Integer b1=2;  
    Integer b2=2;  
    System.out.println(b1==b2);//true,在[-128,127]之间的自动装箱valueOf使用了缓冲机制，不会创建新的对象  
      
    Integer c1=200;  
    Integer c2=200;  
    System.out.println(c1==c2);//false  
    //因此，包装类对象的比较要用equals
    System.out.println(c1.equals(c2));//true
      
    Boolean d1=true;  
    Boolean d2=true;  
    System.out.println(d1==d2);//true

**Integer 缓存源码：**

public static Integer valueOf(int i) {
        
        if (i >= IntegerCache.low && i <= IntegerCache.high)
            return IntegerCache.cache[i + (-IntegerCache.low)];
        return new Integer(i);
    }
    private static class IntegerCache {
        
        static final int low = -128;
        static final int high;
        static {
        
            // high value may be configured by property
            int h = 127;
        }
    }

**`Character` 缓存源码:**

public static Character valueOf(char c) {
        
        if (c <= 127) {
         // must cache
          return CharacterCache.cache[(int)c];
        }
        return new Character(c);
    }
    
    private static class CharacterCache {
        
        private CharacterCache(){
        }
        static final Character cache[] = new Character[127 + 1];
        static {
        
            for (int i = 0; i < cache.length; i++)
                cache[i] = new Character((char)i);
        }
    
    }

如果超出对应范围仍然会去创建新的对象，缓存的范围区间的大小只是在性能和资源之间的权衡。

#### 常用方法 ####

![在这里插入图片描述][8ac49b98cde348ef9ea4ac26b19431ed.png]

1.  Number子类实现的方法

<table> 
 <thead> 
  <tr> 
   <th>Method</th> 
   <th>Description</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td><code>byte byteValue() </code>,<code> short shortValue()</code>,<code> int intValue() </code>,<code> long longValue()</code>,<code> float floatValue()</code>,<code> double doubleValue()</code></td> 
   <td>Converts the value of this <code>Number</code> object to the primitive data type returned.<br><strong>把包装类对象转换成基本数据类型</strong></td> 
  </tr> 
  <tr> 
   <td><code>int compareTo(Byte anotherByte) </code>,<code> int compareTo(Double anotherDouble)</code>,<code> int compareTo(Float anotherFloat)</code>,<code> int compareTo(Integer anotherInteger)</code>,<code> int compareTo(Long anotherLong)</code>,<code> int compareTo(Short anotherShort)</code></td> 
   <td>Compares this <code>Number</code> object to the argument.<br><strong>比较函数</strong></td> 
  </tr> 
  <tr> 
   <td><code>boolean equals(Object obj)</code></td> 
   <td>Determines whether this number object is equal to the argument. The methods return <code>true</code> if the argument is not <code>null</code> and is an object of the same type and with the same numeric value. There are some extra requirements for <code>Double</code> and <code>Float</code> objects that are described in the Java API documentation.<br><strong>是否相等</strong></td> 
  </tr> 
 </tbody> 
</table>

##### Integer的方法 #####

<table> 
 <thead> 
  <tr> 
   <th>Method</th> 
   <th>Description</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td><code>static Integer decode(String s)</code></td> 
   <td>Decodes a string into an integer. Can accept string representations of decimal, octal, or hexadecimal numbers as input.</td> 
  </tr> 
  <tr> 
   <td><code>static int parseInt(String s)</code></td> 
   <td>Returns an integer (decimal only).</td> 
  </tr> 
  <tr> 
   <td><code>static int parseInt(String s, int radix)</code></td> 
   <td>Returns an integer, given a string representation of decimal, binary, octal, or hexadecimal (<code>radix</code> equals 10, 2, 8, or 16 respectively) numbers as input.</td> 
  </tr> 
  <tr> 
   <td><code>String toString()</code></td> 
   <td>Returns a <code>String</code> object representing the value of this <code>Integer</code>.</td> 
  </tr> 
  <tr> 
   <td><code>static String toString(int i)</code></td> 
   <td>Returns a <code>String</code> object representing the specified integer.</td> 
  </tr> 
  <tr> 
   <td><code>static Integer valueOf(int i)</code></td> 
   <td>Returns an <code>Integer</code> object holding the value of the specified primitive.</td> 
  </tr> 
  <tr> 
   <td><code>static Integer valueOf(String s)</code></td> 
   <td>Returns an <code>Integer</code> object holding the value of the specified string representation.</td> 
  </tr> 
  <tr> 
   <td><code>static Integer valueOf(String s, int radix)</code></td> 
   <td>Returns an <code>Integer</code> object holding the integer value of the specified string representation, parsed with the value of radix. For example, if s = “333” and radix = 8, the method returns the base-ten integer equivalent of the octal number 333.</td> 
  </tr> 
 </tbody> 
</table>

public class WrapperMethod {
        
        public static void main(String[] args) {
        
            System.out.println(Integer.MIN_VALUE); //返回最小值
            System.out.println(Integer.MAX_VALUE);//返回最大值
            System.out.println(Character.isDigit('a'));//判断是不是数字
            System.out.println(Character.isLetter('a'));//判断是不是字母
            System.out.println(Character.isUpperCase('a'));//判断是不是大写
            System.out.println(Character.isLowerCase('a'));//判断是不是小写
            System.out.println(Character.isWhitespace('a'));//判断是不是空格
            System.out.println(Character.toUpperCase('a'));//转成大写
            System.out.println(Character.toLowerCase('A'));//转成小写
        }
    }

[8ac49b98cde348ef9ea4ac26b19431ed.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/26/1a14179c55da441196d5bde74fad3d70.png