Language: Java

import java.io.InputStream;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.ArrayDeque;
import java.util.Deque;
import java.util.InputMismatchException;
import java.util.NoSuchElementException;
import java.util.PriorityQueue;
     
public class Main{
    
    static InputReader ir;
    static PrintWriter out;

    static void solve(){
        Deque stk=new ArrayDeque();
	while(ir.hasNext()){
	    String s=ir.next();
	    if(s.charAt(0)=='+'){
		int a=stk.pollFirst();
		int b=stk.pollFirst();
		stk.offerFirst(a+b);
	    }
	    else if(s.charAt(0)=='-'){
		int a=stk.pollFirst();
		int b=stk.pollFirst();
		stk.offerFirst(b-a);
	    }
	    else if(s.charAt(0)=='*'){
		int a=stk.pollFirst();
		int b=stk.pollFirst();
		stk.offerFirst(a+b);
	    }
	    else{
		stk.offerFirst(Integer.parseInt(s));
	    }
        }
    }
 
    public static void main(String[] args) throws Exception {
	ir = new InputReader(System.in);
     	out = new PrintWriter(System.out);
	solve();
	out.flush();
     }
 
    static class InputReader {
	private InputStream in;
	private byte[] buffer=new byte[1024];
	private int curbuf;
	private int lenbuf;

	public InputReader(InputStream in) {this.in=in; this.curbuf=this.lenbuf=0;}
   
	public boolean hasNextByte() {
	    if(curbuf>=lenbuf){
		curbuf= 0;
		try{
		    lenbuf=in.read(buffer);
		}catch(IOException e) {
		    throw new InputMismatchException();
		}
		if(lenbuf<=0) return false;
	    }
	    return true;
	}
  
	private int readByte(){if(hasNextByte()) return buffer[curbuf++]; else return -1;}
 
	private boolean isSpaceChar(int c){return !(c>=33&&c<=126);}
  
	private void skip(){while(hasNextByte()&&isSpaceChar(buffer[curbuf])) curbuf++;}
 
	public boolean hasNext(){skip(); return hasNextByte();}
  
	public String next(){
	    if(!hasNext()) throw new NoSuchElementException();
	    StringBuilder sb=new StringBuilder();
	    int b=readByte();
	    while(!isSpaceChar(b)){
		sb.appendCodePoint(b);
		b=readByte();
	    }
	    return sb.toString();
	}
  
	public int nextInt() {
	    if(!hasNext()) throw new NoSuchElementException();
	    int c=readByte();
	    while (isSpaceChar(c)) c=readByte();
	    boolean minus=false;
	    if (c=='-') {
		minus=true;
		c=readByte();
	    }
	    int res=0;
	    do{
		if(c<'0'||c>'9') throw new InputMismatchException();
		res=res*10+c-'0';
		c=readByte();
	    }while(!isSpaceChar(c));
	    return (minus)?-res:res;
	}
  
	public long nextLong() {
	    if(!hasNext()) throw new NoSuchElementException();
	    int c=readByte();
	    while (isSpaceChar(c)) c=readByte();
	    boolean minus=false;
	    if (c=='-') {
		minus=true;
		c=readByte();
	    }
	    long res = 0;
	    do{
		if(c<'0'||c>'9') throw new InputMismatchException();
		res=res*10+c-'0';
		c=readByte();
	    }while(!isSpaceChar(c));
	    return (minus)?-res:res;
	}
  
	public double nextDouble(){return Double.parseDouble(next());}
  
	public int[] nextIntArray(int n){
	    int[] a=new int[n];
	    for(int i=0;i<n;i++)
		a[i]=nextInt();
	    return a;
	}
  
	public char[][] nextCharMap(int n,int m){
	    char[][] map=new char[n][m];
	    for(int i=0;i<n;i++)
		map[i]=next().toCharArray();
	    return map;
	}
    }
}



Compiler output:
Main.java:20: error: incompatible types: Object cannot be converted to int
		int a=stk.pollFirst();
		                   ^
Main.java:21: error: incompatible types: Object cannot be converted to int
		int b=stk.pollFirst();
		                   ^
Main.java:25: error: incompatible types: Object cannot be converted to int
		int a=stk.pollFirst();
		                   ^
Main.java:26: error: incompatible types: Object cannot be converted to int
		int b=stk.pollFirst();
		                   ^
Main.java:30: error: incompatible types: Object cannot be converted to int
		int a=stk.pollFirst();
		                   ^
Main.java:31: error: incompatible types: Object cannot be converted to int
		int b=stk.pollFirst();
		                   ^
Note: Main.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
6 errors