Building normal self stack in java

Building_normal_stacks.java

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+import java.io.*;
+import java.util.*;
+
+public class Main {
+
+  public static class CustomStack {
+    int[] data;
+    int tos;
+    //tos taken to recieve the data  tos++ and the d[tos]=value
+
+    public CustomStack(int cap) {
+      data = new int[cap];
+      tos = -1;
+
+    }
+
+    int size() {
+        return tos+1;
+
+    }
+
+    void display() {
+        for(int i=tos;i>0;i--)
+        {
+            System.out.print(data[i]+" ");
+        }
+
+        System.out.println();
+
+    }
+
+    void push(int val) {
+        if(tos==data.length-1)
+        {
+            System.out.println("Stack overflow");
+        }
+        else
+        {
+            tos++;
+            data[tos]=val;
+        }
+
+    }
+
+    int pop() {
+        if(tos==-1)
+        {
+            System.out.println("Stack underflow.");
+             return -1;
+        }
+        else
+        {
+            int val=data[tos];
+            tos--;
+            return -1;
+        }
+
+    }
+
+    int top() {
+        if(tos==-1)
+        {
+            System.out.println("Stack overflow");
+             return -1;
+        }
+         else
+        {
+            int val=data[tos];
+            return -1;
+        }
+    }
+  }
+
+  public static void main(String[] args) throws Exception {
+    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
+    int n = Integer.parseInt(br.readLine());
+    CustomStack st = new CustomStack(n);
+
+    String str = br.readLine();
+    while(str.equals("quit") == false){
+      if(str.startsWith("push")){
+        int val = Integer.parseInt(str.split(" ")[1]);
+        st.push(val);
+      } else if(str.startsWith("pop")){
+        int val = st.pop();
+        if(val != -1){
+          System.out.println(val);
+        }
+      } else if(str.startsWith("top")){
+        int val = st.top();
+        if(val != -1){
+          System.out.println(val);
+        }
+      } else if(str.startsWith("size")){
+        System.out.println(st.size());
+      } else if(str.startsWith("display")){
+        st.display();
+      }
+      str = br.readLine();
+    }
+  }
+}