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Value.java
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Value.java
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/**
* Copyright (C) 2022 Czech Technical University in Prague
*
* This program is free software: you can redistribute it and/or modify it under
* the terms of the GNU General Public License as published by the Free Software
* Foundation, either version 3 of the License, or (at your option) any
* later version.
*
* This program is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
* FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
* details. You should have received a copy of the GNU General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*/
package cz.cvut.kbss.ontodriver.model;
import java.util.Objects;
/**
* Represents assertion value.
* <p>
* This class is a base for both property and class assertion values.
*/
public class Value<T> {
/**
* Represents a null value - empty value
*/
private static final Value<?> NULL_VALUE = new NullValue();
private final T value;
private Value() {
this.value = null;
}
public Value(T value) {
Objects.requireNonNull(value);
this.value = value;
}
/**
* Gets this value.
*
* @return Value of the appropriate type
*/
public T getValue() {
return value;
}
/**
* Gets this value as string.
*
* @return Value as string
*/
public String stringValue() {
return value != null ? value.toString() : null;
}
/**
* Returns a Null object for Value.
* <p>
* Since Value requires a non-null value, this method returns a predefined object which represents a null (empty)
* value.
*
* @return Null value
* @see <a href="http://en.wikipedia.org/wiki/Null_Object_pattern">http://en.wikipedia.org/wiki/Null_Object_pattern</a>
*/
@SuppressWarnings("unchecked")
public static <T> Value<T> nullValue() {
return (Value<T>) NULL_VALUE;
}
@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + ((value == null) ? 0 : value.hashCode());
return result;
}
@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
Value<?> other = (Value<?>) obj;
if (value == null) {
return other.value == null;
} else return value.equals(other.value);
}
@Override
public String toString() {
return value != null ? value.toString() : "";
}
private static final class NullValue extends Value<Void> {
private NullValue() {
super();
}
@Override
public String stringValue() {
return "null";
}
}
}