Java: Initialization blocks, static initialization blocks, execution order of construction methods

1. Static initialization block

　　Static initialization blocks are only executed once when the class is first loaded , while static initialization blocks can only assign values to static variables and cannot be assigned to normal member variables.

2. (non-static) initialization block

　(non-static) initialization blocks are executed once each time the instance object is generated , and any variable can be assigned a value.

3. Construction method

　　The construction method executes once each time the instance object is built

4. Execution order: Static initialization block---initialization block----construction method

The test code is as follows:

1  Public classSonextendsFather {2     Static intSnum = 0;3     intnum = 0;4      Public Static voidMain (string[] args) {5SYSTEM.OUT.PRINTLN ("-----");6         NewFather ();7SYSTEM.OUT.PRINTLN ("-----");8         NewSon ();9SYSTEM.OUT.PRINTLN ("-----");TenSystem.out.println ("Snum =" +snum); One     } A      -      PublicSon () { -System.out.println ("Son construction Method"); the     } -      -     Static { -         //num = 1; //Error: Cannot assign a value to a non-static variable +Snum = 1; -System.out.println ("Son static initializer"); +     } A      at     { -System.out.println ("Son initialization block"); -     } -      - } -  in classFather { -     { toSystem.out.println ("Father initialization Block"); +     } -      the      PublicFather () { *System.out.println ("Father construction Method"); $     }Panax Notoginseng      -     Static { theSystem.out.println ("Father static initializer"); +     } A}

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The test results are as follows:

Summary:

2. The static initialization block has the highest precedence, that is, the first execution, and only when the class is loaded for the first time ;
4. Non-static initialization blocks and constructors are executed and executed once each time the object is generated ;
6. The code for a non-static initial block is executed before the class constructor . Therefore, in order to use, we should develop the habit of writing the initialization block before the constructor, which is easy to debug;
8. Static initialization blocks can be used either to initialize static member variables or to execute initialization code ;
10. Non-static initialization blocks can be code reused for multiple overloaded constructors.

Executes only once, when the class is first loaded.

Java: Initialization blocks, static initialization blocks, execution order of construction methods