Java Guide- Added Examples to Built in funcions (#18797)

Added examples of Java.IO package and Java.lang packages

client/src/guide/english/java/built-in-functions/index.md

@@ -11,3 +11,31 @@ import java.io.*;
 ```
 
 These comprise functions which make an otherwise long and hard task easier to do. 
+
+Some examples of ```java.lang.* ``` are:
+
+Generating a random number using Math. In this example a random integer from 1 - 10
+```java
+int randomNumber = (int) (Math.random() * 10) + 1;
+```
+Usage of primitive types such as double, float, int boolean
+
+```java
+int integerNumber = 1;
+double doubleNumber = 0.1d;
+float floatNumber = 0.2f;
+boolean trueOrFalse = false;
+```
+
+
+Classes in ```java.io.* ``` are mainly used for reading and writing of files with diffrent methods such as Input / output streams and buffered readers/ writers.
+And example using a buffered reader is below:
+
+```java
+  File file = new File("C:\\Users\\Desktop\\test.txt"); 
+  BufferedReader br = new BufferedReader(new FileReader(file));   
+  String st; 
+  while ((st = br.readLine()) != null) 
+  System.out.println(st); 
+```
+