Java Conditions and Java Loops, Break Statement, Continue Statement, Java Arrays

Java Conditions and If Statements

Java supports the usual logical conditions from mathematics:

• Less than: a < b
• Less than or equal to: a <= b
• Greater than: a > b
• Greater than or equal to: a >= b
• Equal to a == b
• Not Equal to: a != b

Java has the following conditional statements:

• Use if to specify a block of code to be executed, if a specified condition is true
• Use else to specify a block of code to be executed, if the same condition is false
• Use else if to specify a new condition to test, if the first condition is false
• Use switch to specify many alternative blocks of code to be executed

The if Statement

Use the if statement to specify a block of Java code to be executed if a condition is true.

Syntax

if (condition) {
  // block of code to be executed if the condition is true
}

Note that if is in lowercase letters. Uppercase letters (If or IF) will generate an error.

In the example below, we test two values to find out if 20 is greater than 18. If the condition is true, print some text:

if (20 > 18) {
  System.out.println("20 is greater than 18");
}

We can also test variables:

int x = 20;
int y = 18;
if (x > y) {
  System.out.println("x is greater than y");
}

In the example above we use two variables, x and y, to test whether x is greater than y (using the > operator). As x is 20, and y is 18, and we know that 20 is greater than 18, we print to the screen that "x is greater than y".

The else Statement

Use the else statement to specify a block of code to be executed if the condition is false.

if (condition) {
  // block of code to be executed if the condition is true
} else {
  // block of code to be executed if the condition is false
}

For example,

int time = 20;
if (time < 18) {
  System.out.println("Good day.");
} else {
  System.out.println("Good evening.");
}
// Outputs "Good evening."

In the example above, time (20) is greater than 18, so the condition is false. Because of this, we move on to the else condition and print to the screen "Good evening". If the time was less than 18, the program would print "Good day".

The else if Statement

Use the else if statement to specify a new condition if the first condition is false.

Syntax

if (condition1) {
  // block of code to be executed if condition1 is true
} else if (condition2) {
  // block of code to be executed if the condition1 is false and condition2 is true
} else {
  // block of code to be executed if the condition1 is false and condition2 is false
}

For example,

int time = 22;
if (time < 10) {
  System.out.println("Good morning.");
} else if (time < 20) {
  System.out.println("Good day.");
} else {
  System.out.println("Good evening.");
}
// Outputs "Good evening."

In the example above, time (22) is greater than 10, so the first condition is false. The next condition, in the else if statement, is also false, so we move on to the else condition since condition1 and condition2 is both false - and print to the screen "Good evening".

However, if the time was 14, our program would print “Good day.”

If…Else as Ternary Operator:

There is also a short-hand if else, which is known as the ternary operator because it consists of three operands. It can be used to replace multiple lines of code with a single line. It is often used to replace simple if else statements:

variable = (condition) ? expressionTrue :  expressionFalse;

Instead of writing:

int time = 20;
if (time < 18) {
  System.out.println("Good day.");
} else {
  System.out.println("Good evening.");
}

You can simply write:

int time = 20;
String result = (time < 18) ? "Good day." : "Good evening.";
System.out.println(result);

• Print “Hello World” if x is greater than y.

int x = 50;
int y = 10;
 if(x > y) {
  System.out.println("Hello World");
}

Java Switch Statements

Use the switch statement to select one of many code blocks to be executed.

switch(expression) {
  case x:
    // code block
    break;
  case y:
    // code block
    break;
  default:
    // code block
}

This is how it works:

• The switch expression is evaluated once.
• The value of the expression is compared with the values of each case.
• If there is a match, the associated block of code is executed.
• The break and default keywords are optional, and will be described later in this chapter

The example below uses the weekday number to calculate the weekday name:

int day = 4;
switch (day) {
  case 1:
    System.out.println("Monday");
    break;
  case 2:
    System.out.println("Tuesday");
    break;
  case 3:
    System.out.println("Wednesday");
    break;
  case 4:
    System.out.println("Thursday");
    break;
  case 5:
    System.out.println("Friday");
    break;
  case 6:
    System.out.println("Saturday");
    break;
  case 7:
    System.out.println("Sunday");
    break;
}
// Outputs "Thursday" (day 4)

The break Keyword

When Java reaches a break keyword, it breaks out of the switch block.

This will stop the execution of more code and case testing inside the block.

When a match is found, and the job is done, it’s time for a break. There is no need for more testing.

A break can save a lot of execution time because it “ignores” the execution of all the rest of the code in the switch block.

The default Keyword

The default keyword specifies some code to run if there is no case match:

int day = 4;
switch (day) {
  case 6:
    System.out.println("Today is Saturday");
    break;
  case 7:
    System.out.println("Today is Sunday");
    break;
  default:
    System.out.println("Looking forward to the Weekend");
}
// Outputs "Looking forward to the Weekend"

Note that if the default statement is used as the last statement in a switch block, it does not need a break.

• Another example, Check the output

int day = 2;
switch (day) {
  case 1:
    System.out.println("Saturday");
    break;
  case 2:
    System.out.println("Sunday");
    break;
}

Loops

Loops can execute a block of code as long as a specified condition is reached.

Loops are handy because they save time, reduce errors, and they make code more readable.

Java While Loop

The while loop loops through a block of code as long as a specified condition is true:

while (condition) {
  // code block to be executed
}

In the example below, the code in the loop will run, over and over again, as long as a variable (i) is less than 5:

int i = 0;
while (i < 5) {
  System.out.println(i);
  i++;
}

Note: Do not forget to increase the variable used in the condition, otherwise the loop will never end!

The Do/While Loop

The do/while loop is a variant of the while loop. This loop will execute the code block once, before checking if the condition is true, then it will repeat the loop as long as the condition is true.

Syntax

do {
  // code block to be executed
}
while (condition);

The example below uses a do/while loop. The loop will always be executed at least once, even if the condition is false, because the code block is executed before the condition is tested:

int i = 0;
do {
  System.out.println(i);
  i++;
}
while (i < 5);

Do not forget to increase the variable used in the condition, otherwise the loop will never end!

• Print i as long as i is less than 6.

int i = 1;
 while(i < 6) {
  System.out.println(i); 
  i++;
}

For Loop

When you know exactly how many times you want to loop through a block of code, use the for loop instead of a while loop:

Syntax

for (statement 1; statement 2; statement 3) {
  // code block to be executed
 }

Statement 1 is executed (one time) before the execution of the code block.

Statement 2 defines the condition for executing the code block.

Statement 3 is executed (every time) after the code block has been executed.

The example below will print the numbers 0 to 4:

for (int i = 0; i < 5; i++) {
  System.out.println(i);
}

Statement 1 sets a variable before the loop starts (int i = 0).

Statement 2 defines the condition for the loop to run (i must be less than 5). If the condition is true, the loop will start over again, if it is false, the loop will end.

Statement 3 increases a value (i++) each time the code block in the loop has been executed.

Another Example,

for (int i = 0; i <= 10; i = i + 2) {
  System.out.println(i);
}

For-Each Loop

There is also a “for-each” loop, which is used exclusively to loop through elements in an array:

Syntax

for (type variableName : arrayName) {
  // code block to be executed
}

The following example outputs all elements in the cars array, using a “for-each” loop:

String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
for (String i : cars) {
  System.out.println(i);
}

• Use a for loop to print "Yes" 5 times.

for(int i = 0; i < 5; ) {
  System.out.println();
}

Java Break

You have already seen the break statement used in an earlier chapter of this tutorial. It was used to "jump out" of a switch statement.

The break statement can also be used to jump out of a loop.

This example jumps out of the loop when i is equal to 4:

for (int i = 0; i < 10; i++) {
  if (i == 4) {
    break;
  }
  System.out.println(i);
}

Java Continue

The continue statement breaks one iteration (in the loop), if a specified condition occurs, and continues with the next iteration in the loop.

This example skips the value of 4:

for (int i = 0; i < 10; i++) {
  if (i == 4) {
    continue;
  }
  System.out.println(i);
}

Break and Continue in While Loop

You can also use break and continue in while loops:

Break Example

int i = 0;
while (i < 10) {
  System.out.println(i);
  i++;
  if (i == 4) {
    break;
  }
}

Continue Example

int i = 0;
while (i < 10) {
  if (i == 4) {
    i++;
    continue;
  }
  System.out.println(i);
  i++;
}

• To Stop the loop if i is 5.

for (int i = 0; i < 10; i++) {
  if (i == 5) {   
    break;
  }
  System.out.println(i);
}

Java Arrays

An array is a group of identical data items that are referred by a common name.

Arrays are used to store multiple values in a single variable, instead of declaring separate variables for each value.

In Java, array is dynamic in nature. To declare an array, define the variable type with square brackets:

String[] cars;

We have now declared a variable that holds an array of strings. To insert values to it, we can use an array literal — place the values in a comma-separated list, inside curly braces:

String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};

To create an array of integers:

int[] myNum = {10, 20, 30, 40};

Access the Elements of an Array

You access an array element by referring to the index number. This statement accesses the value of the first element in cars:

String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
System.out.println(cars[0]);
// Outputs Volvo

Note: Array indexes start with 0: [0] is the first element. [1] is the second element, etc.

Change an Array Element

To change the value of a specific element, refer to the index number:

String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
cars[0] = "Open";
System.out.println(cars[0]);
// Now outputs Open instead of Volvo

Properties of an array:

• Array size is fixed, size can not be increased or decreasd on demand.
• Index always begins at 0 and increments by 1.
• No negative indexing.
• Size must be atleast 1.

2D Arrays:

An array with two subscripts which represents rows nd columns is a 2D array.

Syntax:

datatype arrayname[][];
arrayname = new datatype[rows][columns];

For Example,

int a[][];//Array reference
a = new int[3][4];//Memory Allocation

Applications of 2D Arrays:

1. Matrices
2. Image Processing