Java Crash Course: Objects
Objects
Follow along with the instructions at: https://fullstackclouddeveloper.com/c...
Creating a new instance
To create a new instance of a class use the new keyword.
This example create a file object that represents the file sample.txt in the current directory.
File file = new File("sample.txt");
sample.txt is passed into the File constructor that creates a File object from a String filename.
Calling functions on another object
To call a function on another object from the current class we are coding use a . and the function name you want to call.
The nice thing about using an IDE for Java development is the IDE will show you what functions a class has once you put a . after the variable that is holding the instance of that class, such as the variable file below.
Suppose we have a file that wants to find out if the file sample.txt exists.
File file = new File("sample.txt");
if (file.exists()) {
System.out.println("The file exists");
}
Object equality
To check if instance of objects are equal, we need to use the .equals function. Using the == operator will return whether or not the two objects are the same exact instance using its memory identifier.
For example,
String a = new String("hello"); // creates a new String instance
String b = new String("hello"); // creates another new String instance
System.out.println(a == b); // = false
The .equals function is what we use to see if two objects are equal even if they are not the same exact instance.
String a = new String("hello"); // creates a new String instance
String b = new String("hello"); // creates another new String instance
System.out.println(a.equals(b)); // = true
Variable scopes
Class scope
Any variable defined within the class {} is visible to anything within an instance of the class.
Example
public class VariableInClassScope {
// This variable is visible to anything within
// these same brackets.
private String name;
public VariableInClassScope() {
// This is visible within a constructor.
name = "my name";
}
public String getName() {
// This is visible within a function.
return name;
}
private void printNameTenTimes() {
for (int i = 0; i < 10; i++) {
// This is visible within a loop.
System.out.println(name);
}
}
}
Function scope
Any variable defined within the function {} is visible to anything within that function.
Example
public class VariableInClassScope {
public void print() {
// name is not visible here.
System.out.println(name);
}
private void printNameTenTimes() {
// This is visible to anything within these
// same brackets.
String name = "my name";
for (int i = 0; i < 10; i++) {
// name is visible within this loop.
System.out.println(name);
}
}
}
Loop scope
Any variable defined in the for loop, its configuration and within its brackets is visible to anything within its brackets.
Example
public class VariableInClassScope {
private void printZeroToTen() {
for (int i = 0; i < 10; i++) {
int j = i + 1;
// i is visible here.
System.out.println(i);
// j is visible here.
System.out.println(j);
}
// i is not visible here.
System.out.println(i);
// j is not visible here.
System.out.println(j);
}
}
Bracket scope
Any variable defined within the brackets of a code block is visible to anything within its brackets.
public class BracketScope {
public static void main(String[] args) {
{
String letters = "abcdefg";
if (letters.length() > 0) {
// letters is visible here.
char[] chars = letters.toCharArray();
}
}
}
}
About inheritance (extends)
...
About composition (implements)
...
Exercise 6: Implement User Login
For this exercise, use what you have learned so far to implement the classes above. Try to implement these classes without referring back to this lesson as much as possible. Pretend that there is not something to look at and you are implementing this from scratch. Let the "create a user login" purpose guide your implementation and see if it comes out the same.
The recording above includes our attempt at this exercise.
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