Embedded Files
Java Arrays
Java Arrays
Arrays are used to store multiple values in a single variable, instead of declaring separate variables for each value.
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, you could write:
int[] myNum = {10, 20, 30, 40};
Access the Elements of an Array
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:
Example
Example
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
Change an Array Element
To change the value of a specific element, refer to the index number:
Example
Example
cars[0] = "Opel";
Example
Example
String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
cars[0] = "Opel";
System.out.println(cars[0]);
// Now outputs Opel instead of Volvo
Array Length
Array Length
To find out how many elements an array has, use the length property:
Example
Example
String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
System.out.println(cars.length);
// Outputs 4
Loop Through an Array
Loop Through an Array
You can loop through the array elements with the for loop, and use the length property to specify how many times the loop should run.
The following example outputs all elements in the cars array:
Example
Example
String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
for (int i = 0; i < cars.length; i++) {
System.out.println(cars[i]);
}
Loop Through an Array with For-Each
Loop Through an Array with For-Each
There is also a "for-each" loop, which is used exclusively to loop through elements in arrays:
Syntax
Syntax
for (type variable : arrayname) {
...
}
The following example outputs all elements in the cars array, using a "for-each" loop:
Example
Example
String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
for (String i : cars) {
System.out.println(i);
}
The example above can be read like this: for each String element (called i - as in index) in cars, print out the value of i.
If you compare the for loop and for-each loop, you will see that the for-each method is easier to write, it does not require a counter (using the length property), and it is more readable.
Multidimensional Arrays
Multidimensional Arrays
A multidimensional array is an array containing one or more arrays.
To create a two-dimensional array, add each array within its own set of curly braces:
Example
Example
int[][] myNumbers = { {1, 2, 3, 4}, {5, 6, 7} };
myNumbers is now an array with two arrays as its elements.
To access the elements of the myNumbers array, specify two indexes: one for the array, and one for the element inside that array. This example accesses the third element (2) in the second array (1) of myNumbers:
Example
Example
int[][] myNumbers = { {1, 2, 3, 4}, {5, 6, 7} };
int x = myNumbers[1][2];
System.out.println(x); // Outputs 7
We can also use a for loop inside another for loop to get the elements of a two-dimensional array (we still have to point to the two indexes):
Example
Example
public class Main {
public static void main(String[] args) {
int[][] myNumbers = { {1, 2, 3, 4}, {5, 6, 7} };
for (int i = 0; i < myNumbers.length; ++i) {
for(int j = 0; j < myNumbers[i].length; ++j) {
System.out.println(myNumbers[i][j]);
}
}
}
}
Java Arrays
Java Arrays
Arrays are used to store multiple values in a single variable, instead of declaring separate variables for each value.
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, you could write:
int[] myNum = {10, 20, 30, 40};
Access the Elements of an Array
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:
Example
Example
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
Change an Array Element
To change the value of a specific element, refer to the index number:
Example
Example
cars[0] = "Opel";
Example
Example
String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
cars[0] = "Opel";
System.out.println(cars[0]);
// Now outputs Opel instead of Volvo
Array Length
Array Length
To find out how many elements an array has, use the length property:
Example
Example
String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
System.out.println(cars.length);
// Outputs 4
Loop Through an Array
Loop Through an Array
You can loop through the array elements with the for loop, and use the length property to specify how many times the loop should run.
The following example outputs all elements in the cars array:
Example
Example
String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
for (int i = 0; i < cars.length; i++) {
System.out.println(cars[i]);
}
Loop Through an Array with For-Each
Loop Through an Array with For-Each
There is also a "for-each" loop, which is used exclusively to loop through elements in arrays:
Syntax
Syntax
for (type variable : arrayname) {
...
}
The following example outputs all elements in the cars array, using a "for-each" loop:
Example
Example
String[] cars = {"Volvo", "BMW", "Ford", "Mazda"};
for (String i : cars) {
System.out.println(i);
}
The example above can be read like this: for each String element (called i - as in index) in cars, print out the value of i.
If you compare the for loop and for-each loop, you will see that the for-each method is easier to write, it does not require a counter (using the length property), and it is more readable.
Multidimensional Arrays
Multidimensional Arrays
A multidimensional array is an array containing one or more arrays.
To create a two-dimensional array, add each array within its own set of curly braces:
Example
Example
int[][] myNumbers = { {1, 2, 3, 4}, {5, 6, 7} };
myNumbers is now an array with two arrays as its elements.
To access the elements of the myNumbers array, specify two indexes: one for the array, and one for the element inside that array. This example accesses the third element (2) in the second array (1) of myNumbers:
Example
Example
int[][] myNumbers = { {1, 2, 3, 4}, {5, 6, 7} };
int x = myNumbers[1][2];
System.out.println(x); // Outputs 7
We can also use a for loop inside another for loop to get the elements of a two-dimensional array (we still have to point to the two indexes):
Example
Example
public class Main {
public static void main(String[] args) {
int[][] myNumbers = { {1, 2, 3, 4}, {5, 6, 7} };
for (int i = 0; i < myNumbers.length; ++i) {
for(int j = 0; j < myNumbers[i].length; ++j) {
System.out.println(myNumbers[i][j]);
}
}
}
}
Parameters and Arguments
Parameters and Arguments
Information can be passed to methods as parameter. Parameters act as variables inside the method.
Parameters are specified after the method name, inside the parentheses. You can add as many parameters as you want, just separate them with a comma.
The following example has a method that takes a String called fname as parameter. When the method is called, we pass along a first name, which is used inside the method to print the full name:
Example
Example
public class Main {
static void myMethod(String fname) {
System.out.println(fname + " Refsnes");
}
public static void main(String[] args) {
myMethod("Liam");
myMethod("Jenny");
myMethod("Anja");
}
}
// Liam Refsnes
// Jenny Refsnes
// Anja Refsnes
When a parameter is passed to the method, it is called an argument. So, from the example above: fname is a parameter, while Liam, Jenny and Anja are arguments.
Multiple Parameters
Multiple Parameters
You can have as many parameters as you like:
Example
Example
public class Main {
static void myMethod(String fname, int age) {
System.out.println(fname + " is " + age);
}
public static void main(String[] args) {
myMethod("Liam", 5);
myMethod("Jenny", 8);
myMethod("Anja", 31);
}
}
// Liam is 5
// Jenny is 8
// Anja is 31
Note that when you are working with multiple parameters, the method call must have the same number of arguments as there are parameters, and the arguments must be passed in the same order.
Return Values
Return Values
The void keyword, used in the examples above, indicates that the method should not return a value. If you want the method to return a value, you can use a primitive data type (such as int, char, etc.) instead of void, and use the return keyword inside the method:
Example
Example
public class Main {
static int myMethod(int x) {
return 5 + x;
}
public static void main(String[] args) {
System.out.println(myMethod(3));
}
}
// Outputs 8 (5 + 3)
This example returns the sum of a method's two parameters:
Example
Example
public class Main {
static int myMethod(int x, int y) {
return x + y;
}
public static void main(String[] args) {
System.out.println(myMethod(5, 3));
}
}
// Outputs 8 (5 + 3)
You can also store the result in a variable (recommended, as it is easier to read and maintain):
Example
Example
public class Main {
static int myMethod(int x, int y) {
return x + y;
}
public static void main(String[] args) {
int z = myMethod(5, 3);
System.out.println(z);
}
}
// Outputs 8 (5 + 3)
A Method with If...Else
A Method with If...Else
It is common to use if...else statements inside methods:
Example
Example
public class Main {
// Create a checkAge() method with an integer variable called age
static void checkAge(int age) {
// If age is less than 18, print "access denied"
if (age < 18) {
System.out.println("Access denied - You are not old enough!");
// If age is greater than, or equal to, 18, print "access granted"
} else {
System.out.println("Access granted - You are old enough!");
}
}
public static void main(String[] args) {
checkAge(20); // Call the checkAge method and pass along an age of 20
}
}
// Outputs "Access granted - You are old enough!"
Method Overloading
Method Overloading
With method overloading, multiple methods can have the same name with different parameters:
Example
Example
int myMethod(int x)
float myMethod(float x)
double myMethod(double x, double y)
Consider the following example, which have two methods that add numbers of different type:
Example
Example
static int plusMethodInt(int x, int y) {
return x + y;
}
static double plusMethodDouble(double x, double y) {
return x + y;
}
public static void main(String[] args) {
int myNum1 = plusMethodInt(8, 5);
double myNum2 = plusMethodDouble(4.3, 6.26);
System.out.println("int: " + myNum1);
System.out.println("double: " + myNum2);
}
Instead of defining two methods that should do the same thing, it is better to overload one.
In the example below, we overload the plusMethod method to work for both int and double:
Example
Example
static int plusMethod(int x, int y) {
return x + y;
}
static double plusMethod(double x, double y) {
return x + y;
}
public static void main(String[] args) {
int myNum1 = plusMethod(8, 5);
double myNum2 = plusMethod(4.3, 6.26);
System.out.println("int: " + myNum1);
System.out.println("double: " + myNum2);
}
Java Scope
Java Scope
In Java, variables are only accessible inside the region they are created. This is called scope.
Method Scope
Method Scope
Variables declared directly inside a method are available anywhere in the method following the line of code in which they were declared:
Example
Example
public class Main {
public static void main(String[] args) {
// Code here CANNOT use x
int x = 100;
// Code here can use x
System.out.println(x);
}
}
Block Scope
Block Scope
A block of code refers to all of the code between curly braces {}. Variables declared inside blocks of code are only accessible by the code between the curly braces, which follows the line in which the variable was declared:
Example
Example
public class Main {
public static void main(String[] args) {
// Code here CANNOT use x
{ // This is a block
// Code here CANNOT use x
int x = 100;
// Code here CAN use x
System.out.println(x);
} // The block ends here
// Code here CANNOT use x
}
}
A block of code may exist on its own or it can belong to an if, while or for statement. In the case of for statements, variables declared in the statement itself are also available inside the block's scope.
Java Recursion
Java Recursion
Recursion is the technique of making a function call itself. This technique provides a way to break complicated problems down into simple problems which are easier to solve.
Recursion may be a bit difficult to understand. The best way to figure out how it works is to experiment with it.
Recursion Example
Recursion Example
Adding two numbers together is easy to do, but adding a range of numbers is more complicated. In the following example, recursion is used to add a range of numbers together by breaking it down into the simple task of adding two numbers:
Example
Example
Use recursion to add all of the numbers up to 10.
public class Main {
public static void main(String[] args) {
int result = sum(10);
System.out.println(result);
}
public static int sum(int k) {
if (k > 0) {
return k + sum(k - 1);
} else {
return 0;
}
}
}
Example Explained
Example Explained
When the sum() function is called, it adds parameter k to the sum of all numbers smaller than k and returns the result. When k becomes 0, the function just returns 0. When running, the program follows these steps:
10 + sum(9)
10 + ( 9 + sum(8) )
10 + ( 9 + ( 8 + sum(7) ) )
...
10 + 9 + 8 + 7 + 6 + 5 + 4 + 3 + 2 + 1 + sum(0)
10 + 9 + 8 + 7 + 6 + 5 + 4 + 3 + 2 + 1 + 0
Since the function does not call itself when k is 0, the program stops there and returns the result.
Halting Condition
Halting Condition
Just as loops can run into the problem of infinite looping, recursive functions can run into the problem of infinite recursion. Infinite recursion is when the function never stops calling itself. Every recursive function should have a halting condition, which is the condition where the function stops calling itself. In the previous example, the halting condition is when the parameter k becomes 0.
It is helpful to see a variety of different examples to better understand the concept. In this example, the function adds a range of numbers between a start and an end. The halting condition for this recursive function is when end is not greater than start:
Example
Example
Use recursion to add all of the numbers between 5 to 10.
public class Main {
public static void main(String[] args) {
int result = sum(5, 10);
System.out.println(result);
}
public static int sum(int start, int end) {
if (end > start) {
return end + sum(start, end - 1);
} else {
return end;
}
}
}
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