Inverting ifs in JavaParser
Inverting ifs in JavaParser
To answer a question from Kaleem on our Gitter channel, I made a little sample program that turns all if(x!=y) a else b constructs into if(x==y) b else a. I thought it would be a nice example of AST hacking with visitors in practice. Here it is with some comments:
// People posting code samples should stop cutting the imports from them!
import com.github.javaparser.ast.CompilationUnit;
import com.github.javaparser.ast.expr.BinaryExpr;
import com.github.javaparser.ast.expr.Expression;
import com.github.javaparser.ast.stmt.IfStmt;
import com.github.javaparser.ast.stmt.Statement;
import com.github.javaparser.ast.visitor.ModifierVisitor;
import com.github.javaparser.ast.visitor.Visitable;
import com.github.javaparser.utils.CodeGenerationUtils;
import com.github.javaparser.utils.SourceRoot;
import java.io.IOException;
public class Fix {
public static void main(String[] args) throws IOException {
// SourceRoot is a tool that read and writes Java files from packages on a certain root directory.
// In this case the root directory is found by taking the root from the current Maven module,
// with src/main/resources appended.
SourceRoot sourceRoot = new SourceRoot(CodeGenerationUtils.mavenModuleRoot(Fix.class).resolve("src/main/resources"));
// Our sample is in the root of this directory, so no package name.
CompilationUnit cu = sourceRoot.parse("", "Blabla.java");
cu.accept(new ModifierVisitor<Void>() {
@Override
public Visitable visit(IfStmt n, Void arg) {
// I figured out what to get and what to cast simply by looking at the AST in the debugger!
Expression condExpr = n.getCondition();
if (condExpr instanceof BinaryExpr) {
BinaryExpr cond = (BinaryExpr) condExpr;
if (cond.getOperator() == BinaryExpr.Operator.NOT_EQUALS && n.getElseStmt().isPresent()) {
/* It's a good idea to clone nodes that you move around.
JavaParser (or you) might get confused about who their parent is!
*/
Statement thenStmt = n.getThenStmt().clone();
Statement elseStmt = n.getElseStmt().get().clone();
n.setThenStmt(elseStmt);
n.setElseStmt(thenStmt);
cond.setOperator(BinaryExpr.Operator.EQUALS);
}
}
return super.visit(n, arg);
}
}, null);
// This saves back the file we read with the changes we made. Easy!
sourceRoot.saveAll();
}
}