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- Java Operations
Java operations are instructions that can read and write the values of variables, perform arithmetic on variables, and control the program flow.
The core Java operations are:
- Variable Operations
- Variable Assignment
- Variable Reading
- Variable Arithmetics
- Object Instantiation
- Program Flow
- if statements
- switch statements
- for loops
- while loops
- Method Calls
Variable Operations
Java has a set of basic variable manipulation operations. You can assign values to variables, read the value of variables, and perform arithmetics on variable values. Finally, Java also has an operation that allows you to create (instantiate) objects, and assign a reference to the object to a variable.
Variable Assignment
Variable assignment is covered in the text on Java variables, but I will show you a short example here:
int age; age = 25; int yearBorn = 1975;
This example shows two variable assignments. The first example starts by declaring an
int
variable named age
. Then, in the second line it assigns the value 25
to the age
variable. The second example creates an int
variable named yearBorn
and assigns the value 1975
to it in the same statement.Variable Reading
Variable reading is also covered in the text on Java variables, but here is a short example:
String name = "Jakob"; String name2 = name;
This example first creates a variable named
name
and assigns the string value Jakob
to it. In the second line the value of the name
variable is assigned to the name2
variable. The value of the name
variable is read, before it is assigned. This is one way you can read the value of a variable.Variable Arithmetics
Variable arithmetic means performing calculations on variable values. Here are a few examples:
int price = 12; int amount = 23; int totalPrice = price * amount; int discount = 20; //20% int totalAfterDiscount = (totalPrice * (100-discount)) / 100;
In this example two variables called
price
and amount
are declared. Each is a assigned a value. Then a variable called totalPrice
is created, and assigned the value of price * amount
(price multiplied with amount).
Third, a
discount
variable is declared and assigned the value 20
. This variable is to be interpreted as a percentage.
Fourth, the
totalAfterDiscount
variable is declared, and the total price without the 20% discount is calculated, and assigned to totalAfterDiscount
.Object Instantiation
Variables can point both to primitive values (int, float etc.) or to objects. An object is an instance of some class. You instantiate an object of a certain class using the
new
keyword. Here is an example:MyClass myClassInstance = new MyClass();
This example declares a variable of the
MyClass
data type, and then creates a new MyClass
instance and assigns a reference to this instance to myClassInstance
variable.Program Flow
Java also has set of operations targeted at controlling the program flow. Each of these operations are covered in more detail in their own texts, but I will shortly introduce them here.
if statements
Java
if
statements make a decisions between which of two blocks of code to execute. Here is a simple example:int amount = 9; if(amount > 9) { System.out.println("amount is greater than 9"); } else { System.out.println("amount is 9 or less)"); }
This example first declares a variable called
amount
, and assigns the value 9
to it.
What happens next is that the
if
statement compares the value of the amount
variable to 9. If the value is above 9, then the first block of code (inside the { } ) is executed. Else, the second block of code (after the else
keyword) is executed.
The
if
statement can thus be used to select which of two blocks to execute. Actually, the else
block is optional, but the text on Java if explains all that in more detail.switch statements
A switch statement works a bit like an
if
statement, except it can choose between more than two blocks of code to execute. Here is a simple example:int amount = 9; switch(amount) { case 0 : System.out.println("amount is 0"); break; case 5 : System.out.println("amount is 5"); break; case 10 : System.out.println("amount is 10"); break; default : System.out.println("amount is something else"); }
This example first creates a variable named
amount
and assigns the value 9
to it.
Second, the example "switches" on the value of the
amount
variable. Inside the switch
statement are 3 case
statements and a default
statement.
Each
case
statement compares the value of the amount
variable with a constant value. If theamount
variable value is equal to that constant value, the code after the colon (:) is executed. Notice the break
keyword after each statement. If no break
keyword was place here, the execution could continue down the rest of the case
statements. The break
keyword makes execution jump out of the switch
statement.
The
default
statement is executed if no case
statement matched the value of the amount
variable. The default
statement could also be executed if the case
statements before it did not have a break
command in the end.for loops
A
for
loop repeats a block of code as long as some condition is true. Here is a simple example:for(int i=0; i < 10; i++) { System.out.println("I is: " + i); }
This example is a standard
for
loop. Inside the parantheses () after the for
keyword, are three statements separated by semicolon (;).
The first statement declares an
int
variable named i
and assigns it the value 0
. This statement is only executed once, when the for
loop starts.
The second statement compares the value of the
i
variable to the value 10
. If the value of i
is less than 10
, then the for
loop is executed one more time. This statement is executed for each iteration in the for
loop.
The third statement increments the value of
i
. This statement is also executed once per iteration of the for
loop.
The result of this
for
loop is thus, that the body of the loop is executed 10 times. Once for each of the values of i
that are less than 10 (0 to 9).
Java
for
loops are useful when you need to repeat a certain set of operations multiple times. For instance, carry out some set of operations on each element in an array.while loops
Java
while
loops are similar to for
loops. They execute a block of code while a certain condition is true. Here is a simple example:int amount = 0; while(amount < 10) { System.out.println("amount is " + amount); amount++; }
This example first declares a variable named
amount
and assigns the value 0
to it.
Second, the
while
loop is executed. The while
loop executes as long as the comparison inside the parantheses evaluates to true. That is, as long as the amount
variable is less than 10.
Inside the
while
loop the amount
variable is printed out in the first line. In the second line theamount
variable is incremented. The ++
after the variable names means add 1 to the variable. When the amount
variable reaches the value of 10, the condition inside the while
loop parantheses no longer evaluates to true, and the while loop stops.Method Calls
Methods are groups of statements that can be executed as a single statement. Executing a method is also called invoking a method, or calling a method. Here is a simple example of a class that has two methods, where one method calls the other:
public class MyClass() { public void printBoth(String text1, String text2) { print(text1); print(text2); } public void print(String text) { System.out.print(text); } }
This class contains two methods:
printBoth()
and print()
.
Notice how
printBoth()
calls the print()
method two times, each time with a different of the parameters passed to the printBoth()
method.
The
print()
method is thus a reusable block of code that can be called from anywhere. When theprint()
method is finished executing, the program jumps back to after the line that called theprint()
method. Methods can thus be used to jump to different parts of the program, and return from there again.
Methods are covered in more detail in the text on Java methods.