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A constructor is used to instantiate a new instance of a class, constructor has no return type, the constructor name matches exactly the class name. Constructor can be omitted, when it is omitted, JVM will create a default no parameter constructor for you. You can overload constructors with different parameters. You can call other constructor inside a constructor with this(parameter list); Whenever this(parameter list) is called, it has to be the first line in the constructor.
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OCAJP>cat test.java
class test{
String a;
final int b;
public static void main(String...args) {
test t = new test("string only");
System.out.println(t.a + t.b);
t = new test(8);
System.out.println(t.a + t.b);
t = new test("test", 5);
System.out.println(t.a + t.b);
//t = test(); //error: method test in class test cannot be applied to given types
}
public test(String a, int b) {
this.a = a; //this.variableName is refer to instance variable
this.b = b;
}
public test(String a) {
//System.out.println(); //error: call to this must be first statement in constructor
this(a, 0);
}
public test(int b) {
this("string", b);
}
}
OCAJP>javac test.java
OCAJP>java test
string only0
string8
test5
initialize superclass firstly, then static variable and static initializers from top down, then instance variable and instance initializers from top down, finally, the constructor.
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OCAJP>cat test.java
class test{
String a;
final int b;
static boolean choice = true;
static {
choice = false;
System.out.println(choice);
}
public static void main(String...args) {
test t = new test("string only");
System.out.println(t.a + t.b);
}
{
a = "in instance initializer";
System.out.println(a);
}
public test(String a, int b) {
this.a = a; //this.variableName is refer to instance variable
this.b = b;
System.out.println("in constructor public test(String a, int b)");
}
public test(String a) {
//System.out.println(); //error: call to this must be first statement in constructor
this(a, 0);
System.out.println(a);
}
public test(int b) {
this("string", b);
}
}
OCAJP>javac test.java
OCAJP>java test
false
in instance initializer
in constructor public test(String a, int b)
string only
string only0
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