JAVA constructor and java Constructor

JAVA constructor and java Constructor

Let's take a look at two classes: Base and Derived. Note the whenAmISet member variables and the method preProcess ().

Scenario 1: (No constructor for subclass)

class Base {    Base() {        preProcess();    }    void preProcess() {    }}class Derived extends Base {    public String whenAmISet = "set when declared";    void preProcess() {        whenAmISet = "set in preProcess()";    }}public class StaticTest {    public static void main(String[] args) {        Derived d = new Derived();        System.out.println(d.whenAmISet);    }}

After the. java source code is converted into a. class file, it is converted into equivalent code similar to the following:

class Base {    Base() {        preProcess();    }    void preProcess() {    }}class Derived extends Base {    public String whenAmISet;    {whenAmISet = "set when declared";}    void preProcess() {        whenAmISet = "set in preProcess()";    }}public class StaticTest {    public static void main(String[] args) {        Derived d = new Derived();        System.out.println(d.whenAmISet);    }}

The output result is: set when declared.

Scenario 2: (constructor added to subclass)

class Base {    Base() {        preProcess();    }    void preProcess() {    }}class Derived extends Base {    public String whenAmISet = "set when declared";    public Derived() {        whenAmISet = "set in constructor";    }    void preProcess() {        whenAmISet = "set in preProcess()";    }}public class StaticTest {    public static void main(String[] args) {        Derived d = new Derived();        System.out.println(d.whenAmISet);    }}

After the. java source code is converted into a. class file, it is converted into equivalent code similar to the following:

class Base {    Base() {        preProcess();    }    void preProcess() {    }}class Derived extends Base {    public String whenAmISet;    public Derived() {        whenAmISet = "set when declared";        whenAmISet = "set in constructor";    }    void preProcess() {        whenAmISet = "set in preProcess()";    }}public class StaticTest {    public static void main(String[] args) {        Derived d = new Derived();        System.out.println(d.whenAmISet);    }}

Output result: set in constructor

Cause analysis: