4. Decision making statements: if else - ZolotovaNatalia/JavaCourse_2017 GitHub Wiki

If-else Statements

In order to execute a specific part of code you might want some conditions to be fulfilled. For example, if a person is more then 18 years old, he can receive an access to some information, otherwise a message "You are too young to see that". In this case you use a conditional word if. Such situations are very common in real life, that is why in programming languages if-else statement has been developed.

Let's see how it is used in Java:

if(condition is true){
    statement
}else {
    another statement
}

Condition in braces must be a boolean expression. Boolean expressions are expressions that return a boolean value true or false.

Here is a table with boolean expressions:

Operator Name of operator Meanning Example
&& and True if and only if both sides are true warm && sunny
|| or True if either side is true (or if both are true) blue || red
! not Changes true to false, and false to true !tired

These boolean expressions are often used to create a combination of several condition that should be satisfied. Boolean condition can be expressed with mathematical operators:

Operator Name of operator
< less than
<= less than or equal to
== equal to
!= not equal to
>= greater than or equal to
> greater than

For example, if(a > b), if(age > 18). If you want to check the equality condition, use sign ==. For example, if(a == b), if(age == 18).

Let's practice in Java.

Exersices

  1. Compare two int values using mathematical operators
int a = 1;
int b = 5;
if(a > b){
    System.out.println("a > b");
}else if(a < b){
    System.out.println("a < b");
}else {
    System.out.println("a == b");
}

if(a >= b){
    System.out.println("a >= b");
}else {
    System.out.println("a < b");
}

// boolean expression !=
if(a != b){ 
    System.out.println("a != b");
}

//boolean expression ==
if(a == b){ 
    System.out.println("a == b");
}
  1. Compare String variables:
String string1 = "text1";
String string2 = "text2";

System.out.println("string1 has length = " + string1.length());
System.out.println("string2 has length = " + string2.length());

if(string1.length() > string2.length()){
    System.out.println("string1 is longer then string2");
}else {
    System.out.println("string1 is shorter then string2");
}

if(string1.equals(string2)){
    System.out.println("string1 equals to string2");
}else {
    System.out.println("string1 and string2 are different");
}
  1. Boolean expressions: &&, ||:
int a = 1;
int b = 2;
int c = 3;
int d = 4;

// && expression

if((a > b) && (c > d)){
    System.out.println("Both conditions are true: a > b and c > d");
}else {
    System.out.println("One of the conditions is false: a > b or c > d");
}

// || expression

if((a > b) || (c > d)){
    System.out.println("One of the conditions is true: a > b or c > d");
}else {
    System.out.println("Both conditions are false: a > b and c > d");
}
  1. Compare boolean values:
boolean a = true;

if(a){
    System.out.println("a is true");
}else {
    System.out.println("a is false");
}

boolean b = false;

if(!b){
    System.out.println("b is false");
}else {
    System.out.println("b is true");
}

if(a == b){
    System.out.println("a == b");
}else {
    System.out.println("a != b");
}
  1. Conditions in array loops:
int[] a = {1, 2, 3, 4, 5, 6, 7, 8, 9};

for(int i = 0; i < a.length; i++){
    if(a[i] > 4){
        System.out.println("a[" + i + "] = " + a[i]);
    }
}

String[] stringArray = {"a", "ab", "abc", "abcd"};
System.out.println("old stringArray = " + Arrays.toString(stringArray));

int j = 0;
while(j < stringArray.length){
    if(!stringArray[j].equals("abcd")){
        stringArray[j] = "abcd";
    }
    j++;
}
System.out.println("new stringArray = " + Arrays.toString(stringArray));
  1. Write a Java program to find the maximum and minimum value of an array.
  2. Write a Java program to find the index of an array element.
  3. Write a Java program to test the equality of two arrays.
  4. Write a Java program to check if an array of integers without 0 and -1.
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