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Hello everyone , I'm cabbage , You can call me vegetable ( Worthy of the name, it belongs to , ha-ha ~), A sophomore . This article mainly explains Java Interview questions ： Packaging and related knowledge . If you get something after reading the article , You can support bloggers three times in a row ~, Hee hee .

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One 、 Preface

• Come to the point , First, let's take a look at the packaging classes corresponding to the eight basic data types ：

Basic data type	Packaging
char	Character
byte	Byte
short	Short
int	Integer
long	Long
float	Float
double	Double
boolean	Boolean

among Character 、Boolean The parent class is Object, The remaining parent classes are Number

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Two 、 Packing and unpacking

• Automatic packing and unpacking is the automatic conversion between basic data types and packaging classes .JDK1.5 after ,Java Introduced automatic boxing (autoboxing)/ Unpacking (unboxing).
• Automatic boxing ： stay JDK1.5 before , Code Integer i = 10 It's wrong. , Must pass Integer i = new Integer(10) Such statements are used to realize the process of converting basic data types into wrapper classes ; And in the JDK1.5 in the future ,Java It provides the function of automatic packing , The automatic boxing process is performed by calling the wrapper class valueOf() Method , So just Integer i = 10 Such a statement can convert the basic data type into a wrapper class , This is because JVM For us Integer i = Integer.valueOf(10) This kind of operation .

    public void test1() {
    
        Integer a = 10;
        // Equivalent to  Integer a = Integer.valueOf(10);
    }

• Automatic dismantling ： Whenever a value is needed , The object is automatically converted to the basic data type , There is no need to explicitly call intValue()、doubleValue() And other transformation methods . The automatic unpacking process is implemented by calling the wrapper class xxxValue() Method

    public void test1() {
    
        int a = new Integer(10);
        // Equivalent to int a = new Integer(10).intValue();
    }

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3、 ... and 、 Null pointer exception

Now that you know packing and unpacking , So the following code , We can clearly analyze why the null pointer exception is reported ：

    public void test1() {
    
        Integer a = null;
        int b = a;
    }

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Four 、 The use of transformation

The basic data types and the conversion between wrapper classes have been explained earlier , That is, automatic packing and unpacking ; Let's take a look at the basic data types and String Conversion between classes 、String Conversion between class and wrapper class .

1、 Basic data types and String Conversion between classes

• Basic data type to String class ： call valueOf()

    public void test1() {
    
        int a = 3;
        String s = String.valueOf(a);
        System.out.println(s.getClass());
        //class java.lang.String
    }

• String Class to basic data type ： call parseXxx()

    public void test2() {
    
        String s = "12";
        int a = Integer.parseInt(s);
    }

2、String Conversion between class and wrapper class

• String Class to packaging class ： Using constructors

    public void test3() {
    
        String s = "12";
        Integer a = new Integer(s);
        System.out.println(a.getClass());
        //class java.lang.Integer
    }

• Packaging class to String class ：

    public void test4() {
    
        Integer a = new Integer(12);
        String s = a.toString();
        String s1 = Integer.toString(a);
        System.out.println(s.getClass());
        System.out.println(s1.getClass());
        //class java.lang.String
    }

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5、 ... and 、 Classic interview questions

Try to write , See if you can ：

    @Test
    public void test5() {
    
        Integer i = new Integer(1);
        Integer j = new Integer(1);
        System.out.println(i == j);
        Integer m = 1;
        Integer n = 1;
        System.out.println(m == n);
        Integer x = 128;
        Integer y = 128;
        System.out.println(x == y);
    }

Output results ：false、true、false

Why is that ？ Recommend an article here , It's really great ：https://www.cnblogs.com/dolphin0520/p/3780005.html

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6、 ... and 、 summary

End with a picture ~

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Thank you for reading , Progress together , Hee hee ~