Classes
Parameters
p76 The variables passed as method's parameters has to match the type.
/**
* The variables passed as method's parameters has to match the type.
* For example, if we pass a string as math.sum() param we get compile error.
*/
package com.minte9.basics.variables;
public class Parameters {
public static void main(String[] args) {
Math math = new Math();
int sum = math.sum(1,2); // math.sum(1, "2"); // compile error
System.out.println(
"Sum(1,2) = " + sum
);
}
}
class Math {
public int sum(int n1, int n2) {
return n1 + n2;
}
}
// Output: Sum(1,2) = 3
Default values
p85 Object instance variables always get a default value.
/**
* Class instance variables always have default value.
* Local variable does not have a default value, they must initialized.
*/
package com.minte9.basics.variables;
public class DefaultValues {
public static void main(String[] args) {
new Values().showValues();
}
}
class Values {
int a; // 0
float b; // 0.0
boolean c; // false
Values v; // null
public void showValues() {
String local_variable = "a"; // no default value
System.out.println("int a = " + a);
System.out.println("float b = " + b);
System.out.println("boolean c = " + c);
System.out.println("object v = " + v);
System.out.println("String local_var = " + local_variable);
}
}
/*
int a = 0
float b = 0.0
boolean c = false
object v = null
String local_var = a
*/
Pass by value
p77 Variables in Java are passed by value (a copy), not by reference.
/**
* Variables in Java are passed by value (copy), not by reference
*
* In this example, first variable x = 7 bits are copied (00000111).
* Second this copy goes in z variable.
* Third, after variable z changes, we can see that x is not changed.
*/
package com.minte9.basics.variables;
public class PassedByValue {
public static void main(String[] args) {
int x = 7; // 00000111
int z = x; // x bits are copied in z
System.out.println("x == z " + (x == z));
z = 0;
System.out.println("z changed / x doesn't change");
System.out.println("x != z " + (x != z));
}
}
/*
x == z true
z changed / x doesn't change
x != z true
*/
Setter
p79 Setter and getter naming convention are an important Java standard.
/**
* Setter and getter JavaBeans specifications:
*
* The setter method for foo must be called setFoo()
* The gettter method for xIndex must be called getxIndex()
*/
package com.minte9.basics.variables;
public class Setter {
public static void main(String[] args) {
Dog dog = new Dog();
dog.setName("Rex");
System.out.println(dog.getName()); // Rex
}
}
class Dog {
private String name;
public void setName(String x) {
name = x;
}
public String getName() {
return name;
}
}
➥ Questions
