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JAutoDiff
There is a newer version: 1.16
package com.accelad.math.nilgiri.autodiff;

import java.util.List;

import com.accelad.math.nilgiri.AbstractIdentityFactory;
import com.accelad.math.nilgiri.Field;


public class Variable> extends DifferentialFunction {

    private X m_x;
    private AbstractIdentityFactory m_factory;
    private String m_name;

    private PreEvaluator preEvaluator;

    protected Variable(String i_name, X i_v, AbstractIdentityFactory i_factory) {
        this(i_name, i_v, i_factory, null);
    }

    protected Variable(String i_name, X i_v, AbstractIdentityFactory i_factory,
            PreEvaluator preEvaluator) {
        this.preEvaluator = preEvaluator;
        setName(i_name);
        if (i_v != null && i_factory != null) {
            m_x = i_v;
            m_factory = i_factory;
        } else {
            throw new IllegalArgumentException("Input not null value.");
        }
    }

    protected AbstractIdentityFactory factory() {
        return m_factory;
    }

    private void setName(String i_name) {
        if (i_name != null) {
            m_name = i_name;// new String(i_name);
        } else {
            throw new IllegalArgumentException("Input not null value.");
        }
    }

    public String getName() {
        return m_name;
    }

    public void set(X i_v) {
        if (i_v != null) {
            m_x = i_v;
        } else {
            throw new IllegalArgumentException("Input not null value.");
        }
    }

    @Override
    public boolean isVariable() {
        return true;
    }

    @Override
    public X getValue() {
        if (preEvaluator != null) {
            preEvaluator.update(this);
        }
        return m_x;
    }

    @Override
    public double getReal() {
        if (preEvaluator != null) {
            preEvaluator.update(this);
        }
        return m_x.getReal();
    }

    @Override
    public Constant diff(Variable i_v) {
        return (this == i_v) ? new One(m_factory) : new Zero(m_factory);
    }

    @Override
    public String toString() {
        return getName() + ":" + getValue();
    }

    @Override
    public String getFormula(List> variables) {
        variables.add(this);
        return getName();
    }

    @Override
    public DifferentialFunction div(DifferentialFunction i_v) {
        return (i_v == this) ? new One(m_factory) : super.mul(i_v.inverse());
    }

}