Control structures allow us to control the flow of our programs. The if statement is the simplist of these. As implied by the name, an if statement will allow something to occur if a boolean condition is met.
An if statement takes the following form.
if(boolean expression){
//the code between these curly brackets are executed
//if the boolean expression in parenthesis is true
}
The if-else statement allows you to handle conditions that evaluate to true as well as false. Just as we saw in the example above, the if portion of this statement will occur if a boolean condition is met, otherwise the else portion of the statement will be executed. Consider the example below,
if(boolean expression){
//the code between these curly brackets are executed
//if the boolean expression in parenthesis is true
}else{
//the code between these curly brackets are executed
//if the boolean expression in parenthesis is false
}
In a previous lesson we learned that numerical expressions can be evaluated using either of the following expressions.
== !=
Consider the following examples,
int i1 = 2;
int i2 = 3;
if(i1 == i2){
//the code between these curly brackets will not be executed
//because i1 is not the same as i2
}else{
//the code between these curly brackets will be executed
//because i1 is not the same as i2
}
int i1 = 2;
int i2 = 3;
if(i1 != i2){
//the code between these curly brackets will be executed
//because i1 is not the same as i2
}else{
//the code between these curly brackets will not be executed
//because i1 is not the same as i2
}
Numerical expressions can also be combined using either the OR and AND operators,
|| &&
For example,
int i1 = 2;
int i2 = 3;
if(( i1 < 10 ) && ( i2 == 5)){
//the code between these curly brackets will not be executed
//because the condition evaluates to false
}else{
//the code between these curly brackets will be executed
//because the condition evaluates to false
}
int i1 = 2;
int i2 = 3;
if(( i1 < 10 ) || ( i2 == 5)){
//the code between these curly brackets will be executed
//because condition evaluates to true
}else{
//the code between these curly brackets will not be executed
//because the condition evaluates to true
}
In the previous examples the == notation was used to compare values. However, this notation is only valid for comparing primitive type variables. Recall that String variables are not primitives - they are object variables. We will discuss the difference between primitive and object type variables later, but for now just recognize that String variables (or any object type variable) cannot be compared using the == operator. String variables can only be compared by using the equals method illustrated below,
String c = "cat";
String d = "dog";
boolean b = c.equals(d);//compares d to c and assigns the result to b
System.out.println(b);//prints false
b = !c.equals(d);
//reverses the comparison with the not operator
System.out.println(b);//prints true
How the equals method can be combined with if-else statements is illustrated below,
String s1 = "a";
String s2 = "b";
String s3 = "b";
if(( s1.equals(s2)) && ( s2.equals(s3))){
//the code between these curly brackets will not be executed
//because the condition evaluates to false
}else{
//the code between these curly brackets will be executed
//because the condition evaluates to false
}
if(( s1.equals(s2)) || ( s2.equals(s3))){
//the code between these curly brackets will be executed
//because the condition evaluates to true
}else{
//the code between these curly brackets will not be executed
//because the condition evaluates to true
}
Multiple if statements can be combined with if-else. This is illustrated below,
Scanner s = new Scanner(System.in);
System.out.println("What is your grade?");
int theGrade = s.nextInt();
if(theGrade>=90){
System.out.println("You got an A!");
}else if(theGrade>=80){
System.out.println("You got a B!");
}else if(theGrade>=70){
System.out.println("You got a C!");
}else if(theGrade>=60){
System.out.println("You got a D!");
}else{
System.out.println("You failed!");
}
A nested if or if-else statement refers to an if or if-else statement inside of an if-else statement. Consider the following example,
Scanner s = new Scanner(System.in);
System.out.println("What is your grade?");
int theGrade = s.nextInt();
if(theGrade>=90){
if(theGrade<94){
System.out.println("you got an A-");
}else{
System.out.println("you got an A");
}
}else if(theGrade>=80){
if(theGrade>86){
System.out.println("you got a B+");
}else if(theGrade<84){
System.out.println("you got a B-");
}else{
System.out.println("you got a B");
}
}else{
System.out.println("you got another grade");
}
Curly brackets are not needed if only one line of code is in the if or else parts. Likewise, the absence of brackets implies only one line of code in the if-else parts.
This is illustrated below.
int groovyDude=37;
if(groovyDude == 37)
groovyDude++;//this line is executed
System.out.println(groovyDude);//38 is printed
int groovyDude=105;
if(groovyDude == 37)
groovyDude++;//this line is not executed
System.out.println(groovyDude);//105 is printed
int groovyDude=105;
if(groovyDude == 37)
groovyDude++;//this line is not executed
else
groovyDude--;//this line is executed
System.out.println(groovyDude);//104 is printed