Added a NonNullSupplier that throws an exception when a null value is supplied.

Author: Darkhax

src/main/java/net/darkhax/bookshelf/lib/function/NonNullSupplier.java

@@ -0,0 +1,82 @@
+package net.darkhax.bookshelf.lib.function;
+
+import java.util.function.Supplier;
+
+/**
+ * An implementation of Supplier that wraps another supplier with a null check. If the wrapped
+ * supplier returns a null value a NullReturnedException will be raised.
+ *
+ * @param <T> The type of value returned by the supplier.
+ */
+public class NonNullSupplier<T> implements Supplier<T> {
+    
+    /**
+     * The internal supplier being wrapped. This is the source of returned values.
+     */
+    private final Supplier<T> internal;
+    
+    /**
+     * A supplier that returns the error message used when a null value is encountered.
+     */
+    private final Supplier<String> errorSupplier;
+    
+    private NonNullSupplier(Supplier<T> internal, Supplier<String> errorSupplier) {
+        
+        this.internal = internal;
+        this.errorSupplier = errorSupplier;
+    }
+    
+    @Override
+    public T get () {
+        
+        final T value = this.internal.get();
+        
+        if (value == null) {
+            
+            throw new NullReturnedException(this.errorSupplier.get());
+        }
+        
+        return value;
+    }
+    
+    /**
+     * Wraps the provided supplier with a null check. If the wrapped supplier returns null a
+     * NullReturnedException will be raised.
+     * 
+     * @param <T> The type of value returned by the supplier.
+     * @param toWrap The supplier to wrap.
+     * @return A supplier that has been wrapped with a null check.
+     */
+    public static <T> Supplier<T> from (Supplier<T> toWrap) {
+        
+        return NonNullSupplier.from(toWrap, () -> "The returned value was null.");
+    }
+    
+    /**
+     * Wraps the provided supplier with a null check. If the wrapped supplier returns null a
+     * NullReturnedException will be raised.
+     * 
+     * @param <T> The type of value returned by the supplier.
+     * @param toWrap The supplier to wrap.
+     * @param errorSupplier A supplier that generates the exception message when a null value
+     *        is returned from the wrapped supplier.
+     * @return A supplier that has been wrapped with a null check.
+     */
+    public static <T> Supplier<T> from (Supplier<T> toWrap, Supplier<String> errorSupplier) {
+        
+        return new NonNullSupplier<>(toWrap, errorSupplier);
+    }
+    
+    /**
+     * An exception that is thrown when a NonNullSupplier would return a null value.
+     */
+    public static class NullReturnedException extends RuntimeException {
+        
+        private static final long serialVersionUID = -1848463236953236103L;
+        
+        private NullReturnedException(String message) {
+            
+            super(message);
+        }
+    }
+}