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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.dataflow.cfg.node;

import com.sun.source.tree.Tree;

import org.checkerframework.checker.nullness.qual.Nullable;
import org.checkerframework.dataflow.qual.SideEffectFree;
import org.checkerframework.javacutil.TreeUtils;

import java.util.Collection;
import java.util.Collections;
import java.util.Objects;

/**
 * A node for the unary 'nullchk' operation (generated by the Java compiler):
 *
 *  *   <*nullchk*>expression
 * 
 */
public class NullChkNode extends Node {
    /** The entire tree of the null check */
    protected final Tree tree;

    /** The operand of the null check */
    protected final Node operand;

    /**
     * Constructs a {@link NullChkNode}.
     *
     * @param tree the nullchk tree
     * @param operand the operand of the null check
     */
    public NullChkNode(Tree tree, Node operand) {
        super(TreeUtils.typeOf(tree));
        assert tree.getKind() == Tree.Kind.OTHER;
        this.tree = tree;
        this.operand = operand;
    }

    public Node getOperand() {
        return operand;
    }

    @Override
    public Tree getTree() {
        return tree;
    }

    @Override
    public  R accept(NodeVisitor visitor, P p) {
        return visitor.visitNullChk(this, p);
    }

    @Override
    public String toString() {
        return "(+ " + getOperand() + ")";
    }

    @Override
    public boolean equals(@Nullable Object obj) {
        if (!(obj instanceof NumericalPlusNode)) {
            return false;
        }
        NumericalPlusNode other = (NumericalPlusNode) obj;
        return getOperand().equals(other.getOperand());
    }

    @Override
    public int hashCode() {
        return Objects.hash(NullChkNode.class, getOperand());
    }

    @Override
    @SideEffectFree
    public Collection getOperands() {
        return Collections.singletonList(getOperand());
    }
}