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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.constantpropagation;

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

import java.util.Objects;

import org.checkerframework.dataflow.analysis.AbstractValue;

public class Constant implements AbstractValue {

    /** What kind of abstract value is this? */
    protected Type type;

    /** The value of this abstract value (or null) */
    protected /*@Nullable*/ Integer value;

    public enum Type {
        CONSTANT, TOP, BOTTOM,
    }

    public Constant(Type type) {
        assert !type.equals(Type.CONSTANT);
        this.type = type;
    }

    public Constant(Integer value) {
        this.type = Type.CONSTANT;
        this.value = value;
    }

    public boolean isTop() {
        return type.equals(Type.TOP);
    }

    public boolean isBottom() {
        return type.equals(Type.BOTTOM);
    }

    public boolean isConstant() {
        return type.equals(Type.CONSTANT);
    }

    public Integer getValue() {
        assert isConstant();
        return value;
    }

    public Constant copy() {
        if (isConstant()) {
            return new Constant(value);
        }
        return new Constant(type);
    }

    @Override
    public Constant leastUpperBound(Constant other) {
        if (other.isBottom()) {
            return this.copy();
        }
        if (this.isBottom()) {
            return other.copy();
        }
        if (other.isTop() || this.isTop()) {
            return new Constant(Type.TOP);
        }
        if (other.getValue().equals(getValue())) {
            return this.copy();
        }
        return new Constant(Type.TOP);
    }

    @Override
    public boolean equals(Object obj) {
        if (obj == null || !(obj instanceof Constant)) {
            return false;
        }
        Constant other = (Constant) obj;
        return type == other.type && Objects.equals(value, other.value);
    }

    @Override
    public int hashCode() {
        return type.hashCode() + (value != null ? value.hashCode() : 0);
    }

    @Override
    public String toString() {
        switch (type) {
        case TOP:
            return "T";
        case BOTTOM:
            return "-";
        case CONSTANT:
            return value.toString();
        }
        assert false;
        return "???";
    }
}