96 lines
2.4 KiB
Java
96 lines
2.4 KiB
Java
/**
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* This class implements something that
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* probably is just a value but may be nothing.
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* We will never return null in this class, but
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* we may return something that contains nothing
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* where the nothing is a null.
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*
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* @author XXX
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* @version CS2030S AY22/23 Semester 1
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*/
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class Probably<T> {
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private final T value;
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private static final Probably<?> NONE = new Probably<>(null);
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/**
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* Private constructor, can only be invoked inside.
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* This is called a factory method. We can only
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* create this using the two public static method.
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*
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* @return The shared NOTHING.
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*/
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private Probably(T value) {
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this.value = value;
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}
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/**
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* It is probably nothing, no value inside.
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*
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* @return The shared NOTHING.
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*/
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public static <T> Probably<T> none() {
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@SuppressWarnings("unchecked")
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Probably<T> res = (Probably<T>) NONE;
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return res;
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}
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/**
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* It is probably just the given value.
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* Unless the value is null, then nothing is
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* given again.
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*
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* @param value Probably this is the value
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* unless it is null then we say
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* that there is no
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* @return The given value or nothing but
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* never null.
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*/
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public static <T> Probably<T> just(T value) {
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if (value == null) {
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return none();
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}
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return (Probably<T>) new Probably<>(value);
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}
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/**
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* Check for equality between something that
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* is probably a value but maybe nothing.
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*
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* @param obj The other value to be compared.
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* @return True if the two values are equal,
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* false otherwise.
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*/
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@Override
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public boolean equals(Object obj) {
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if (obj == this) {
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return true;
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}
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if (obj instanceof Probably<?>) {
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Probably<?> some = (Probably<?>) obj;
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if (this.value == some.value) {
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return true;
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}
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if (this.value == null || some.value == null) {
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return false;
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}
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return this.value.equals(some.value);
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}
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return false;
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}
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/**
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* String representation of something that
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* is probably a value but maybe nothing.
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*
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* @return The string representation.
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*/
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@Override
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public String toString() {
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if (this.value == null) {
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return "<>";
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} else {
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return "<" + this.value.toString() + ">";
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}
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}
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} |