Java programming language

Control structures

Selective structures

  

Selective or conditional structures let us choose among different paths in our code depending on a given condition (or conditions) that must be checked. In this document we’ll see the selective structures provided by Java.

1. The “if” clause

You can use the if basic structure to run a piece of code if a given condition is true:

if (age >= 18)
{
    System.out.println("You are old enough");
}

Alternatively, you can use if..else structure to choose among a given path (determined by a condition) or its opposite:

if (age >= 18)
{
    System.out.println("You are old enough");
}
else
{
    System.out.println("You are not adult yet");
}

Finally, if there is more than two paths to choose among, we can join if .. else if .. else if .. else structures to choose among several paths depending on the condition relative to each one. Only one path will be chosen.

if (number > 0)
{
    System.out.println("It is positive");
}
else if (number < -10)
{
    System.out.println("It is under -10");
}
else
{
    System.out.println("It is between -10 and 0");
}

The condition in each if or else if clause can be either a simple condition (like the ones shown in previous examples) or a complex condition, joined by logical operators && and/or ||:

if (age >= 18 && age <= 30)
{
    System.out.println("You are between 18 and 30");
}

2. The “switch” clause

Besides, there’s a switch clause that lets us evaluate the value of a simple variable or expression. Each of the possible values of this expression can be represented with a case clause. Finally, we can use a default case to represent any other possible value that has not been covered by previous case clauses.

The data managed in the switch clause must be a primitive type; strings are NOT allowed in early versions of Java (Java 6 and earlier). We need to add a break instruction at the end of each case to exit the switch clause; otherwise, the program keeps running the instruction of next case clause. In other languages, such as C#, the break instruction is compulsory in the case clauses that are not empty, but this does not happen in Java, so we must take care of this situation.

switch(number)
{
    case 0:	System.out.println("It is 0"); break;
    case 1: System.out.println("It is 1");
    case 2: System.out.println("It is 2"); break;
    default: System.out.println("Unknown number");
}

In previous example, if number is 1, it would output the messages “It is 1” and “It is 2”, since there is no break clause at case 1.

Exercise 1:

Create a program called MarkCheck that asks the user to enter 3 marks. The program must print one of these messages, depending on the mark values:

• All marks are greater or equal than 4
• Some marks are not greater or equal than 4
• No mark is greater or equal than 4

Exercise 2:

Create a program called GramOunceConverter2 that will be an improved version of a previous exercise. In this case, the user will type a weight (float), and a unit (g for grams, o for ounces). Then, depending on the unit chosen, the program will convert the weight to the opposite unit. For instance, if the user types a weight of 33 and chooses o as unit, then the program must convert 33 ounces to grams. You must solve this program using a switch structure. If the unit is other than g or o, then the program must output an error message: “Unexpected unit”, with no final result.