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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.framework.source;

/*>>>
import org.checkerframework.checker.nullness.qual.*;
*/

import org.checkerframework.javacutil.ErrorReporter;

import javax.annotation.processing.ProcessingEnvironment;
import javax.lang.model.util.Elements;
import javax.lang.model.util.Types;

import com.sun.source.tree.CompilationUnitTree;
import com.sun.source.util.TreePath;
import com.sun.source.util.TreePathScanner;
import com.sun.source.util.Trees;

/**
 * An AST visitor that provides a variety of compiler utilities and interfaces
 * to facilitate type-checking.
 */
public abstract class SourceVisitor
        extends TreePathScanner {

    /** The {@link Trees} instance to use for scanning. */
    protected final Trees trees;

    /** The {@link Elements} helper to use when scanning. */
    protected final Elements elements;

    /** The {@link Types} helper to use when scanning. */
    protected final Types types;

    /** The root of the AST that this {@link SourceVisitor} will scan. */
    protected CompilationUnitTree root;


    /**
     * Creates a {@link SourceVisitor} to use for scanning a source tree.
     *
     * @param checker the checker to invoke on the input source tree
     */
    public SourceVisitor(SourceChecker checker) {
        // Use the checker's processing environment to get the helpers we need.
        ProcessingEnvironment env = checker.getProcessingEnvironment();

        this.trees = Trees.instance(env);
        this.elements = env.getElementUtils();
        this.types = env.getTypeUtils();

        // Install the SourceChecker as the error handler
        // TODO: having this static state is ugly. Use the context to instantiate.
        ErrorReporter.setHandler(checker);
    }

    /*
     * Set the CompilationUnitTree to be used during any visits.
     * For any later calls of {@see com.sun.source.util.TreePathScanner.scan(TreePath, P)},
     * the CompilationUnitTree of the TreePath has to be equal to {@code root}.
     */
    public void setRoot(CompilationUnitTree root) {
        this.root = root;
    }

    /*
     * Entry point for a type processor: the TreePath leaf is
     * a top-level type tree within root.
     */
    public void visit(TreePath path) {
        this.scan(path, null);
    }
}