Java method
last modified January 27, 2024
In this article we cover Java methods.
In object oriented programming, we work with objects. Objects are basic building blocks of a program. Objects consists of data and methods. Methods change the state of the objects created. They are the dynamic part of the objects; the data is the static part.
Java method definition
A method is a code block containing a series of statements. Methods must be declared within a class. It is a good programming practice that methods do only one specific task. Methods bring modularity to programs. Proper use of methods brings the following advantages:
- Reducing duplication of code
- Decomposing complex problems into simpler pieces
- Improving clarity of the code
- Reuse of code
- Information hiding
Java method characteristics
Basic characteristics of methods are:
- Access level
- Return value type
- Method name
- Method parameters
- Parentheses
- Block of statements
Access level of methods is controlled with access modifiers. They set the
visibility of methods. They determine who can call the method. Methods may
return a value to the caller. If our method returns a value, we declare its data
type. If not, we use the void keyword to indicate that our method
does not return any value.
Method parameters are surrounded by parentheses and separated by commas. Empty
parentheses indicate that the method requires no parameters. The method block is
surrounded with {} characters. The block contains one or more
statements that are executed when the method is invoked. It is legal to
have an empty method block.
Java method signature
A method signature is a unique identification of a method for the Java compiler. The signature consists of a method name, and the type and kind (value, reference, or output) of each of its formal parameters. Method signature does not include the return type.
Java method names
Any legal character can be used in the name of a method. By convention, method names begin with a lowercase letter. The method names are verbs or verbs followed by adjectives or nouns. Each subsequent word starts with an uppercase character. The following are typical names of methods in Java:
- execute
- findId
- setName
- getName
- checkIfValid
- testValidity
Java method example
We start with a simple example.
package com.zetcode;
class Base {
public void showInfo() {
System.out.println("This is Base class");
}
}
public class ShowInfoMethod {
public static void main(String[] args) {
Base bs = new Base();
bs.showInfo();
}
}
We have a showInfo method that prints the name of
its class.
class Base {
public void showInfo() {
System.out.println("This is Base class");
}
}
Each method must be defined inside a class. It must have a name. In our case,
the name is showInfo. The keywords that precede the name of the
method are access specifier and the return type. Parentheses follow the name of
the method. They may contain parameters of the method. Our method does not take
any parameters.
public static void main(String[] args) {
...
}
This is the main method. It is the entry point to each console or
GUI Java application. The method takes a string array of arguments.
Base bs = new Base(); bs.showInfo();
We create an instance of the Base class. We call the
showInfo method upon the object. We say that the method is an
instance method because it needs an instance to be called. The method is called
by specifying the object instance, followed by the member access operator — the
dot, followed by the method name.
Java method parameters
A parameter is a value passed to the method. Methods can take one or more parameters. If methods work with data, we must pass the data to the methods. This is done by specifying them inside the parentheses. In the method definition, we must provide a name and type for each parameter.
package com.zetcode;
class AddValues {
public int addTwoValues(int x, int y) {
return x + y;
}
public int addThreeValues(int x, int y, int z) {
return x + y + z;
}
}
public class Addition {
public static void main(String[] args) {
AddValues a = new AddValues();
int x = a.addTwoValues(12, 13);
int y = a.addThreeValues(12, 13, 14);
System.out.println(x);
System.out.println(y);
}
}
In the above example, we have an AddValues class which has two
methods. One of them takes two parameters, the other one takes three parameters.
public int addTwoValues(int x, int y) {
return x + y;
}
The addTwoValues method takes two parameters. These parameters
have int type. The method also returns an integer to the caller.
We use the return keyword to return a value from the
method.
public int addThreeValues(int x, int y, int z) {
return x + y + z;
}
The addThreeValues is similar to the previous method, but it takes
three parameters.
int x = a.addTwoValues(12, 13);
We call the addTwoValues method of the AddValues
object. It takes two values. These values are passed to the method. The
method returns a value which is assigned to the x variable.
Variable number of arguments
A method can take variable number of arguments. For this we use ellipsis.
package com.zetcode;
public class SumOfValues {
public static int sum(int...vals) {
int sum = 0;
for (int val : vals) {
sum += val;
}
return sum;
}
public static void main(String[] args) {
int s1 = sum(1, 2, 3);
int s2 = sum(1, 2, 3, 4, 5);
int s3 = sum(1, 2, 3, 4, 5, 6, 7);
System.out.println(s1);
System.out.println(s2);
System.out.println(s3);
}
}
We create a sum method which can take variable number of arguments.
The method calculates the sum of integers passed to the method.
int s1 = sum(1, 2, 3); int s2 = sum(1, 2, 3, 4, 5); int s3 = sum(1, 2, 3, 4, 5, 6, 7);
We call the sum method three times. In each case, we pass different
number of parameters to the method.
public static int sum(int...vals) {
...
}
The sum method can take variable number of integer values. All
values are added to an array.
int sum = 0;
for (int val : vals) {
sum += val;
}
return sum;
We compute the sum of the values and return the calculated sum.
$ java com.zetcode.SumOfValues 6 15 28
Passing arguments by value
In Java, arguments are always passed by value to methods. When we pass primitive types, the copies of the values are sent to the methods. In case of objects, copies of the references are handed to the methods.
Java does not support passing arguments by reference, like C# or C++.
package com.zetcode;
class Cat {}
class Dog {}
public class PassByValue {
private static void tryChangeInteger(int x) {
x = 15;
}
private static void tryChangeObject(Object o) {
Dog d = new Dog();
o = d;
}
public static void main(String[] args) {
int n = 10;
tryChangeInteger(n);
System.out.println(n);
Cat c = new Cat();
tryChangeObject(c);
System.out.println(c.getClass());
}
}
The example shows that it is not possible to change the values of primitive types and the references to objects inside methods.
private static void tryChangeInteger(int x) {
x = 15;
}
The value of the passed variable is copied to the local variable x.
Assigning a new value to the x variable does not affect the outside
variable.
private static void tryChangeObject(Object o) {
Dog d = new Dog();
o = d;
}
The same applies for objects. We are passing copies of references to methods.
The o is a local variable that refers to the Dog object. The object
defined outside the tryChangeObject is not affected.
int n = 10; tryChangeInteger(n); System.out.println(n);
We define the n variable and pass it to the
tryChangeInteger method. Later, we print it to check if it
was modified.
Cat c = new Cat(); tryChangeObject(c); System.out.println(c.getClass());
We define a Cat object and pass it to the
tryChangeObject method.
$ java com.zetcode.PassByValue 10 class com.zetcode.Cat
From the output we can see that neither the primitive value nor the object were modified.
Method overloading in Java
Method overloading allows the creation of several methods with the same name which differ in the type of the input.
What is method overloading good for? Qt library gives a nice example for the
usage. The QPainter class has three methods to draw a rectangle.
Their name is drawRect and their parameters differ. One takes a
reference to a floating point rectangle object, another takes a reference to an
integer rectangle object, and the last one takes four parameters:
x, y, width, and height.
If the C++ language, which is the language in which Qt is developed, didn't have
method overloading, the creators of the library would have to name the methods
like drawRectRectF, drawRectRect,
drawRectXYWH. The solution with method overloading is more elegant.
package com.zetcode;
class Sum {
public int getSum() {
return 0;
}
public int getSum(int x) {
return x;
}
public int getSum(int x, int y) {
return x + y;
}
}
public class Overloading {
public static void main(String[] args) {
Sum s = new Sum();
System.out.println(s.getSum());
System.out.println(s.getSum(5));
System.out.println(s.getSum(5, 10));
}
}
We have three methods called setSum. They differ in input
parameters.
public int getSum(int x) {
return x;
}
This one takes one parameter.
System.out.println(s.getSum()); System.out.println(s.getSum(5)); System.out.println(s.getSum(5, 10));
We call all three methods. All methods have the same name. The compiler knows which method to call on the grounds of the method input.
$ java com.zetcode.Overloading 0 5 15
And this is what we get when we run the example.
Java recursion
Recursion, in mathematics and computer science, is a way of defining methods in which the method being defined is applied within its own definition. In other words, a recursive method calls itself to do its job. Recursion is a widely used approach to solve many programming tasks. Every problem solved using a recursion can be worked out with the iteration too.
A typical example is the calculation of a factorial.
package com.zetcode;
public class Recursion {
static int factorial(int n) {
if (n == 0) {
return 1;
} else {
return n * factorial(n - 1);
}
}
public static void main(String[] args) {
System.out.println(factorial(6));
System.out.println(factorial(15));
}
}
In this code example, we calculate the factorial of two numbers.
return n * factorial(n - 1);
Inside the body of the factorial method, we call the factorial method with a modified argument. The function calls itself. This is the essence of the recursion algorithm.
$ java com.zetcode.Recursion 720 2004310016
These are the results.
Java method scope
A variable declared inside a method has a method scope. The scope of a name is the region of program within which it is possible to refer to the entity declared by the name without the qualification of the name. A variable which is declared inside a method has a method scope. It is also called a local scope. The variable is valid only in this particular method.
package com.zetcode;
class Test {
int x = 1;
public void exec1() {
System.out.println(this.x);
System.out.println(x);
}
public void exec2() {
int z = 5;
System.out.println(x);
System.out.println(z);
}
}
public class MethodScope {
public static void main(String[] args) {
Test ts = new Test();
ts.exec1();
ts.exec2();
}
}
In this example, we have the x variable defined outside the instance
methods. The variable has a class scope. It is valid everywhere inside
the definition of the Test class, e.g. between its curly brackets.
public void exec1() {
System.out.println(this.x);
System.out.println(x);
}
The x variable, also called the x field, is an instance variable. It is accessible
through this keyword. It is also valid inside the exec1
method and can be referred by its bare name. Both statements refer to the same variable.
public void exec2() {
int z = 5;
System.out.println(x);
System.out.println(z);
}
The x variable can be accessed also in the exec2 method. The z
variable is defined in the exec2 method. It has a method scope. It is valid
only in this method.
$ java com.zetcode.MethodScope 1 1 1 5
A variable defined inside a method has a local/method scope. If a local variable
has the same name as an instance variable, it shadows the instance
variable. The instance variable is still accessible inside the method with
this.
package com.zetcode;
class Test {
int x = 1;
public void exec() {
int x = 3;
System.out.println(this.x);
System.out.println(x);
}
}
public class Shadowing {
public static void main(String[] args) {
Test t = new Test();
t.exec();
}
}
We declare an instance variable x. We declare another
x variable inside the exec method. Both variables
have the same name, but they are not in conflict because they reside in
different scopes.
System.out.println(this.x); System.out.println(x);
The variables are accessed differently. The x variable defined
inside the method, also called a local variable x, is simply
accessed by its name. The instance variable can be referred by using with
this.
$ java com.zetcode.Shadowing 1 3
Java static methods
Static methods are called without an instance of the object. To call a static method, we use the name of the class and the dot operator. Static methods can only work with static variables. Static methods are often used to represent data or calculations that do not change in response to object state. An example is a math library which contains static methods for various calculations.
We use the static keyword to declare a static method or a static
variable. When no static modifier is present, the method is said to be an
instance method. We cannot use this keyword in static methods; it
can be used in instance methods only.
package com.zetcode;
class Basic {
static int id = 2321;
public static void showInfo() {
System.out.println("This is Basic class");
System.out.format("The Id is: %d%n", id);
}
}
public class StaticMethod {
public static void main(String[] args) {
Basic.showInfo();
}
}
In our code example, we define a static ShowInfo method.
static int id = 2321;
A static method can only work with static variables. Static variables are not available to instance methods.
public static void showInfo() {
System.out.println("This is Basic class");
System.out.format("The Id is: %d%n", id);
}
This is our static ShowInfo method. It works with a static
id member.
Basic.showInfo();
To invoke a static method, we do not need an object instance. We call the method by using the name of the class and the dot operator.
$ java com.zetcode.StaticMethod This is Basic class The Id is: 2321
Java hiding methods
In case of static methods, a method in the derived class with the same signature as in the base class hides the one in the base class. The method to be called is determined at compile time. This process is referred to as early or static binding.
package com.zetcode;
class Base {
public static void showInfo() {
System.out.println("This is Base class");
}
}
class Derived extends Base {
public static void showInfo() {
System.out.println("This is Derived class");
}
}
public class Hiding {
public static void main(String[] args) {
Base.showInfo();
Derived.showInfo();
}
}
We have two classes: Derived and Base. The
Derived class inherits from the Base class. Both have
a method called showInfo.
class Derived extends Base {
public static void showInfo() {
System.out.println("This is Derived class");
}
}
The static class method showInfo of the Derived
class hides the showInfo method of the Base class.
Base.showInfo(); Derived.showInfo();
We call the showInfo method for both classes. Each
class calls its own method.
$ java com.zetcode.Hiding This is Base class This is Derived class
Java overriding methods
Overriding happens when we create an instance method of a derived class with the same signature and return type as an instance method in the base class. The method to be executed is determined at runtime. The determining of the method to be executed at runtime is called late or dynamic binding.
We might want to use the @Override annotation that instructs
the compiler that we intend to override a method in the superclass. It helps
prevent some programming errors.
package com.zetcode;
class Base {
public void showInfo() {
System.out.println("This is Base class");
}
}
class Derived extends Base {
@Override
public void showInfo() {
System.out.println("This is Derived class");
}
}
public class Overriding {
public static void main(String[] args) {
Base[] objs = { new Base(), new Derived(), new Base(),
new Base(), new Base(), new Derived() };
for (Base obj : objs) {
obj.showInfo();
}
}
}
We create an array of the Base and Derived
objects. We go through the array and invoke the showInfo
method upon all of
them.
@Override
public void showInfo() {
System.out.println("This is Derived class");
}
Here we are overriding the showInfo method of the
Base class.
Base[] objs = { new Base(), new Derived(), new Base(),
new Base(), new Base(), new Derived() };
Here we create an array of Base and Derived objects.
Note that we used the Base type in our array declaration. The
Derived class can be converted to the Base class,
because it inherits from it. The opposite is not true. The only way to have
defferent objects in one array is to use a type which is shared by all objects.
for (Base obj : objs) {
obj.showInfo();
}
We traverse the array and call showInfo on all objects in the array.
The method to be called is determined at runtime.
$ java com.zetcode.Overriding This is Base class This is Derived class This is Base class This is Base class This is Base class This is Derived class
With the super keyword, it is possible to call the overriden
method.
package com.zetcode;
class Base {
public void showInfo() {
System.out.println("This is Base class");
}
}
class Derived extends Base {
@Override
public void showInfo() {
System.out.println("This is Derived class");
}
public void showBaseInfo() {
super.showInfo();
}
}
public class Overriding2 {
public static void main(String[] args) {
Derived d = new Derived();
d.showBaseInfo();
}
}
In the example, we call the showInfo of the Base class
with super.
public void showBaseInfo() {
super.showInfo();
}
Here we call the immediate parent's showInfo
method.
Java final methods
A final method cannot be overriden or hidden by derived classes.
This is used to prevent unexpected behavior from a subclass altering a method
that may be crucial to the function or consistency of the class.
package com.zetcode;
class Base {
public void f1() {
System.out.println("f1 of the Base");
}
public final void f2() {
System.out.println("f2 of the Base");
}
}
class Derived extends Base {
@Override
public void f1() {
System.out.println("f1 of the Derived");
}
// @Override
// public void f2() {
//
// System.out.println("f2 of the Derived");
// }
}
public class FinalMethods {
public static void main(String[] args) {
Base b = new Base();
b.f1();
b.f2();
Derived d = new Derived();
d.f1();
d.f2();
}
}
In this example we have a final method f2 in the
Base class. This method cannot be overriden.
public final void f2() {
System.out.println("f2 of the Base");
}
The f2 method is declared to be final. No overloading
is possible.
@Override
public void f1() {
System.out.println("f1 of the Derived");
}
In the Derived class, we can override the f1 method of
the Base class. We also use the @Override annotation
to inform the compiler that we are overriding a method.
// @Override
// public void f2() {
//
// System.out.println("f2 of the Derived");
// }
These lines are commented, because otherwise the code example would not compile. The compiler would give the following error: Exception in thread "main" java.lang.VerifyError: class com.zetcode.Derived overrides final method f2.
d.f2();
Since it is not possible to override a final method, the above line will
call the f2 method of the Base class.
$ java com.zetcode.FinalMethods f1 of the Base f2 of the Base f1 of the Derived f2 of the Base
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In this article we have covered Java methods.
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