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The Checker Framework enhances Java’s type system to make it more powerful and useful. This lets software developers detect and prevent errors in their Java programs. The Checker Framework includes compiler plug-ins ("checkers") that find bugs or verify their absence. It also permits you to write your own compiler plug-ins.

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package org.checkerframework.checker.units;

import org.checkerframework.checker.units.qual.UnknownUnits;
import org.checkerframework.common.basetype.BaseTypeChecker;
import org.checkerframework.common.basetype.BaseTypeVisitor;
import org.checkerframework.framework.source.Result;
import org.checkerframework.framework.type.AnnotatedTypeMirror;

import com.sun.source.tree.CompoundAssignmentTree;
import com.sun.source.tree.ExpressionTree;
import com.sun.source.tree.Tree.Kind;

/**
 * Units visitor.
 *
 * Ensure consistent use of compound assignments.
 */
public class UnitsVisitor extends BaseTypeVisitor {
    public UnitsVisitor(BaseTypeChecker checker) {
        super(checker);
    }

    @Override
    public Void visitCompoundAssignment(CompoundAssignmentTree node, Void p) {
        ExpressionTree var = node.getVariable();
        ExpressionTree expr = node.getExpression();
        AnnotatedTypeMirror varType = atypeFactory.getAnnotatedType(var);
        AnnotatedTypeMirror exprType = atypeFactory.getAnnotatedType(expr);

        Kind kind = node.getKind();

        if ( (kind == Kind.PLUS_ASSIGNMENT || kind == Kind.MINUS_ASSIGNMENT)) {
            if (!atypeFactory.getTypeHierarchy().isSubtype(exprType, varType)) {
                checker.report(Result.failure("compound.assignment.type.incompatible",
                        varType, exprType), node);
            }
        } else if (exprType.getAnnotation(UnknownUnits.class) == null) {
            // Only allow mul/div with unqualified units
            checker.report(Result.failure("compound.assignment.type.incompatible",
                    varType, exprType), node);
        }

        return null; // super.visitCompoundAssignment(node, p);
    }

}