Is my search algorithim correct

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Java

algorithm

1)I am lost with the search algorithm and how to output the result.
2)How to output the average and the grades below 50.

I do not know how to go about the linear search algorithm.

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.Scanner;

public class pdemo {
    private String[] names;
    private double[] grade;
    public int Nos;
    private String num;
    public char a=0,b=0,c=0,x=0;
    private BufferedReader stdin;
    Scanner on = new Scanner(System.in);
    public static final int NOT_FOUND = -1;
    
    public pdemo()
            throws IOException {
        stdin = new BufferedReader(new InputStreamReader(System.in));
        System.out.printf("%s\n", "Welcome Enter the Number of student taking This Couse");
        num = stdin.readLine();    Nos = Integer.parseInt(num);
        names = new String[Nos];    grade=new double[Nos];
    }
    
    public void inputNames()
            throws IOException {
        for (int i = 0;
                i < names.length;
                i++) {
            System.out.print("Enter name: ");
            names[i] = stdin.readLine();
        }
    }

    public void inputGrades()
            throws IOException {
        for(int i=0;
                i<grade.length;
                i++) {
            System.out.println("Enter the grade ");
            // grade[i] =Double.parseDouble(num);
            grade[i]=on.nextDouble();
        }
    }
    
    public void trav()
            throws IOException {
        for (int da=0;
                da<names.length;
                da++) {
            System.out.println( da+ ":"+ names[da]+" :"+grade[da]);
        }
    }

    public void sele()
            throws IOException {
        System.out.println("CHOSE FORM THE LIST OF OPERATION");
        System.out.printf("%s\n %s\n %s\n ",
                "a = Search for a particular Student",
                "b=Find the average of all Student",
                "c= Find all student with the marks blow 50");
        // for(int i=0; i < num; i++)
        String in = stdin.readLine();
    }
}

Here are the codes I am having problems with.

linear search algorithm

public void search(String[] names, double[]  grade,String num,int i)
        throws IOException {
    if(a==a) {
        System.out.println("What is the name of the student u are looking for?");
        names[i] = stdin.readLine();
    }
    {
        int low = 0;
        int high = names.length - 1;
        int mid;
        while( low <= high ) {
            mid = ( low + high ) / 2;
            if (names[mid].compareTo( num)< 0)
                low = mid + 1;
            else if (names[ mid ].compareTo( num) > 0)
                high = mid - 1;
            else
                return mid;
        }

        return NOT_FOUND; // NOT_FOUND = -1
    }
}

System.out.println(names[i]+""+grade[i]);
}
public void average(String [] names, double [] grade,int num) {
    if (b==b) {
        double ave=0;
        for(int i=0;
                i < num;i++) {
            ave+=grade[i];
        }
        System.out.println(ave);
    }
}

public void below(String [] names, double [] grade,int num)
{
    {
        for(int i=0;
                i <
                num;i++) {
            if(grade[i] < 50) {
                System.out.println(names[i]+grade[i]);
            }
        }
    }
}

import java.io.*;
public class Main {
    public static int n;

    /**
     * @param args the command line arguments
     */
    private static BufferedReader stdin = new BufferedReader( new InputStreamReader( System.in ) );

    public static void main(String[] args)
            throws IOException {
        pdemo demo = new pdemo();
        demo.inputNames();
        demo.inputGrades();
        demo.trav();
        demo.sele();
    {
}

I have a problem here too:

demo.search(args, grade, null, n);
demo.average(args, grade, null, n);
demo.below(args, grade, null, n);