Question

What kind of error is incompatible types? Compilation error, runtime error, or semantic error? Why does incompatible types error happen? How would you fix this error using wildcards? Language Java.

incompatible types: List<String> cannot be converted to List<Object>

Answer 1

Error is an illegal operation performed by the user which results in the abnormal working of the program. Programming errors often remain undetected until the program is compiled or executed. Some of the errors inhibit the program from getting compiled or executed. Thus errors should be removed before compiling and executing.

Java Error incompatible types occurred when a compiler found a variable and expression whose data type is not compatible to perform an operation on them.

Example :

For this we have a class name Incompatobletype.Inside the main method we declare a variable name roll num and sec int and char data type. Checklist condition is used to evaluate the expression. The assignment operator = is used to show you an incompatible type error in java rather than using = = in assignment operator. The Java Compiler show an error in the assignment operator printed by System.out.println.

Incompatitabletype.java

 //Incompatitabletype.java public class Icompatobletype { public static void main(String[] args) { int rollnum = 76; char sec; if (rollnum = 90) { sec = 'A'; } else { sec = 'B'; } System.out.println("Section with this rollno is: = " + sec); } } 

To resolve this error give if (rollnum = =90)instead of if (rollnum = 90)

Compilation Error : Compile Time Errors are those errors which prevent the code from running because of an incorrect syntax such as a missing semicolon at the end of a statement or a missing bracket, class not found, etc. These errors are detected by the java compiler and an error message is displayed onto the screen while compiling. Compile Time Errors are sometimes also referred to as Syntax errors. These kind of errors are easy to spot and rectify because the java compiler finds them for you. The compiler will tell you which piece of code in the program got in trouble and its best guess as to what you did wrong. Usually, the compiler indicates the exact line where the error is, or sometimes the line just before it, however, if the problem is with incorrectly nested braces, the actual error may be at the beginning of the block. In effect, syntax errors represent grammatical errors in the use of the programming language.

 // Example :Misspelled variable name or method names class MisspelledVar { public static void main(String args[]) { int a = 40, b = 60; // Declared variable Sum with Capital S int Sum = a + b; // Trying to call variable Sum // with a small s ie. sum System.out.println( "Sum of variables is " + sum); } } //Compilation Error in java code: prog.java:14: error: cannot find symbol + sum); ^ symbol: variable sum location: class MisspelledVar 1 error //Example : Missing semicolons class PrintingSentence { public static void main(String args[]) { String s = "GeeksforGeeks"; // Missing ';' at the end System.out.println("Welcome to " + s) } } Compilation Error in java code: prog.java:8: error: ';' expected System.out.println("Welcome to " + s) ^ 1 error // Example: Missing parenthesis, square brackets, or curly braces class MissingParenthesis { public static void main(String args[]) { System.out.println("Printing 1 to 5 \n"); int i; // missing parenthesis, should have // been for(i=1; i<=5; i++) for (i = 1; i <= 5; i++ { System.out.println(i + "\n"); } // for loop ends } // main ends } // class ends //Compilation Error in java code: prog.java:10: error: ')' expected for (i = 1; i <= 5; i++ { ^ 1 error // Example: Incorrect format of selection statements or loops class IncorrectLoop { public static void main(String args[]) { System.out.println("Multiplication Table of 7"); int a = 7, ans; int i; // Should have been // for(i=1; i<=10; i++) for (i = 1, i <= 10; i++) { ans = a * i; System.out.println(ans + "\n"); } } } //Compilation Error in java code: prog.java:12: error: not a statement for (i = 1, i <= 10; i++) { ^ prog.java:12: error: ';' expected for (i = 1, i <= 10; i++) { ^ 2 errors

Note: run each class given in example codes separately.

Run Time error : Run Time errors occur or are detected during the execution of the program. Sometimes these are discovered when the user enters an invalid data or data which is not relevant. Runtime errors occur when a program does not contain any syntax errors but asks the computer to do something that the computer is unable to reliably do. During compilation, the compiler has no technique to detect these kinds of errors. It is the JVM (Java Virtual Machine) which detects it while the program is running. To handle the error during the run time we can put our error code inside the try block and catch the error inside the catch block.

For example: if the user inputs a data of string format when the computer is expecting an integer, there will be a runtime error.

Example 1: Runtime Error caused by dividing by zero

// Java program to demonstrate Runtime Error //Example Division by zero error    class DivByZero {     public static void main(String args[])     {         int var1 = 15;         int var2 = 5;         int var3 = 0;         int ans1 = var1 / var2;            // This statement causes a runtime error,         // as 15 is getting divided by 0 here         int ans2 = var1 / var3;            System.out.println("Division of va1" + " by var2 is: "+ ans1);         System.out.println("Division of va1"+ " by var3 is: "+ ans2);     } }

Exception in thread "main" java.lang.ArithmeticException: / by zeroRunTime Error in java code:

 at DivByZero.main(File.java:14) 

Example 2: Runtime Error caused by Assigning/Retrieving Value from an array using an index which is greater than the size of the array

class RTErrorDemo {     public static void main(String args[])     {         int arr[] = new int[5];         // Array size is 5         // whereas this statement assigns         // value 250 at the 10th position.         arr[9] = 250;         System.out.println("Value assigned! ");     } }

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 9RunTime Error in java code:

 at RTErrorDemo.main(File.java:10) 

Semantic Error or Logical Error : A logic error is when your program compiles and executes, but does the wrong thing or returns an incorrect result or no output when it should be returning an output. These errors are detected neither by compiler nor by JVM. The Java system has no idea what your program is supposed to do, so it provides no additional information to help you find the error. Logical errors are also called Semantic Errors. These errors are caused due to an incorrect idea or concept used by a programmer while coding. Syntax errors are grammatical errors whereas, logical errors are errors arising out of an incorrect meaning.

1. For example: if a programmer accidentally adds two variables when he or she meant to divide them, the program will give no error and will execute successfully but with an incorrect result.

   Example: Accidentally using an incorrect operator on the variables to perform an operation (Using '/' operator to get the modulus instead using '%')

    public class LErrorDemo {     public static void main(String[] args)     {         int num = 789;         int reversednum = 0;         int remainder;         while (num != 0) {             // to obtain modulus % sign should have been used instead of /             remainder = num / 10;             reversednum                 = reversednum * 10                   + remainder;             num /= 10;         }         System.out.println("Reversed number is " + reversednum);     } }

   
   Output:

    Reversed number is 7870 

   Example: Displaying the wrong message

   class IncorrectMessage {     public static void main(String args[])     {         int a = 2, b = 8, c = 6;         System.out.println("Finding the largest number \n");            if (a > b && a > c)             System.out.println(a + " is the largest Number");         else if (b > a && b > c)             System.out.println(b + " is the smallest Number");            // The correct message should have         // been System.out.println         //(b+" is the largest Number");         // to make logic         else             System.out.println(c + " is the largest Number");     } }

   
   Output:

   Finding the largest number 8 is the smallest Number