Dead code in Java
There are several meanings for the term "dead code." Some people use the word to describe code or instructions stored in memory that can never be run at run-time. Dead code, as used in various computer programming contexts, refers to a portion of a program's source code that runs but whose output is never used in another computation. Dead code costs memory and computational resources when it runs.
Example:
double div_by_0(double x, double y) { if (y == 0) return 1; else return 0; return x/y; // Unreachable code (dead code) }
Explanation:
The mathematical expression a/b follows two return statements within an if-else block. One of the return statements will be executed because of the if-else combination, making the last return statement an orphan that cannot be reached. Any code after the return statements cannot be performed because code execution is linear. There is no conditional expression enclosing the return statement.
Causes for Dead code:
There are several causes for dead code, and here are some of the reasons:
- Unused Variables and Methods: Declared variables and methods become dead code if never used or called.
- Unreachable Code: Code that follows an endless loop, throw statement, or return statement without a break is regarded as unreachable code.
- Conditional Statements with Constant Conditions: Code inside the unreachable branch becomes dead code if a conditional statement continuously evaluates to true or false.
- Unused Imports: Dead code is sometimes caused by import declarations for classes or packages not utilized in the code.
- Debugging constructs and other nonfunctional code
- Reusable components that are designed for different run-time scenarios
Dead Code Detection:
- Integrated Development Environment (IDE): To detect dead code, use the built-in features in your IDE. For example, you can enable code inspections for unused code in Eclipse or IntelliJ IDEA.
- Polyspace tools: You can find unreachable or dead code in your software using polyspace tools. As a result, testing operations focused on robustness and complete code coverage take less time and money. The comprehensive data that Polyspace Solutions offers about the behaviour of software during run-time validates the presence of dead code and helps you in tracking control and data flow to possible sources.
- Code Coverage Tools:
In addition, Polyspace products integrate well with code coverage tools like VectorCast, which can import the results of Polyspace product verification and use them to complement their code coverage solution.
Code coverage is a metric to quantify whether a software application has been thoroughly tested and has a lower chance of containing bugs. Structural code coverage is primarily used to indicate when adequate testing has been achieved.
Dead-code elimination:
Dead-code elimination, also known as dead-code removal, dead-code stripping, or dead-code strip, is a compiler optimization used in compiler theory to eliminate dead code or code that does not affect the program's outcome. Eliminating such code has multiple advantages: it makes the program smaller, which is beneficial in some situations, and it enables the program to run faster by preventing unnecessary operations from being executed. Through program structure simplification, it can also facilitate additional optimizations. Dead code is irrelevant to the program and can never be completed (also known as unreachable code). It also affects variables that are written to but never read again.
An everyday use of dead-code elimination is an alternative to optional code inclusion via a preprocessor. Consider the following code.
int main(void) { int a = 5; int b = 6; int c; c = a * (b / 2); if (0) { /* DEBUG */ printf("%d\n", c); } return c; }
Dead-code elimination would eliminate the code inside the if statement from the optimized program because it can never be executed because the expression 0 will always be evaluated as false. This method is frequently used in debugging to activate code blocks selectively; by employing an optimizer with dead-code elimination built in, a preprocessor is not required to carry out the same function.
Implementation
Let us try to understand the concept of dead code using a java program
File name: Deadcode.java
public class Deadcode { public String deadcode() { System.out.println("This is a deadcode() method "); return "Hello"; System.out.println("This is an unreachable code "); } public static void main(String args[]) { new Deadcode().deadcode(); } }
Output:
ERROR! javac /tmp/z16VqrNFae/Deadcode.java /tmp/z16VqrNFae/Deadcode.java:8: error: unreachable statement System.out.println("This is an unreachable code "); ^ /tmp/z16VqrNFae/Deadcode.java:9: error: missing return statement } ^ 2 errors