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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.LiteralTree;
import com.sun.source.tree.Tree;

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

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

/**
 * A node for the null literal.
 *
 *  *   null
 * 
 */
public class NullLiteralNode extends ValueLiteralNode {

    /**
     * Create a new NullLiteralNode.
     *
     * @param t the tree for the literal value
     */
    public NullLiteralNode(LiteralTree t) {
        super(t);
        assert t.getKind() == Tree.Kind.NULL_LITERAL;
    }

    @Override
    public Void getValue() {
        return (Void) tree.getValue();
    }

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

    @Override
    public boolean equals(@Nullable Object obj) {
        if (this == obj) {
            return true;
        }
        if (!(obj instanceof NullLiteralNode)) {
            return false;
        }
        // super method compares values
        return super.equals(obj);
    }

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