java-predicates

collection of predefined predicates for all basic types

Maven

Add the Jitpack repository to your build file

<repositories>
    <repository>
        <id>jitpack.io</id>
        <url>https://jitpack.io</url>
    </repository>
</repositories>

Release artifact

<dependency>
    <groupId>com.github.zrdj</groupId>
    <artifactId>java-predicates</artifactId>
    <version>0.3.0</version>
</dependency>

Motivation

The motivation behind this project is to achieve a more readable and maintainable way of chaining checks on all kind of basic types (primitives, strings, arrays, etc ) we use in our everyday code, by providing predefined Predicate<T> implementations.

So lets be naive and try to check that a given String conforms to a valid .de and .com URL.

    public void doSomethingWhenUrlIsValid(final String url) {
    if (url != null
            && !url.isEmpty()
            && url.startsWith("http://")
            && (url.endsWith(".de") || url.endsWith(".com"))) {
        doSomethingWithThatUrl();
    } else {
        throw new IllegalArgumentException("invalid url: " + url);
    }
}

Basically this code has two issues, a major and a minor one. the major one is that whenever the condition on what a valid URL is changes, you have to change the if statement. the minor one is that its not as fluent as it could be for the reader.

Now lets use a separate method and the Predicate<T> interface from Java 8 to fix our major issue ...

    public void doSomethingWhenUrlIsValid(final String url) {
    if (isValidUrl().test(url)) {
        doSomethingWithThatUrl();
    } else {
        throw new IllegalArgumentException("invalid url: " + url);
    }
}

public Predicate<String> isValidUrl() {
    return string -> string != null
            && !string.isEmpty()
            && string.startsWith("http://")
            && (string.endsWith(".de") || string.endsWith(".com"));
}

... and rewrite isValidUrl() method using our fluent Predicates interface ...

    public Predicate<String> isValidUrl() {
    final Predicates checking = new Predicates.Default();
    final StringPredicates it = checking.string();

    return checking.that().valid().when().string().isNotNull()
            .and(it.isNotEmpty())
            .and(it.startsWith("http://"))
            .and(it.endsWith(".de").or(it.endsWith(".com")));
}