Float. This is a floating-point, 4-byte number. In Java a float is like a double but only has 4 bytes (a double has twice as many bytes).

double

With this type, we can store numbers with a decimal place. A float can be cast to a double without any special syntax. But we must be careful when converting from a double to a float.

Float examples. Here are two syntax forms for creating a float value. We can use (float) to specify the type. And the lowercase "f" specifies a float literal.

public class Program {
    public static void main(String[] args) {

        // Use a float variable.
        // ... Uses a float cast.
        float number = (float) 10.5;
        System.out.println(number);

        // This syntax also works.
        float number2 = 10.75f;
        System.out.println(number2);

        // A float is 4 bytes.
        System.out.println(Float.BYTES);
    }
}
10.5
10.75
4

Max, min values. Floats can store a wide range of values. But with floats we also find a "normal values" concept. A normal value is within a "balanced range" of values.

public class Program {
    public static void main(String[] args) {
        float max = Float.MAX_VALUE;
        float minNormal = Float.MIN_NORMAL;
        float min = Float.MIN_VALUE;

        // Print some float values.
        System.out.println(max);
        System.out.println(minNormal);
        System.out.println(min);
    }
}
3.4028235E38
1.17549435E-38
1.4E-45

Float, double arguments. Suppose we have a method that receives a double argument. We can pass floats or doubles to it—these values are all "widened" to match the method signature.

public class Program {

    static void print(double value) {
        System.out.println(value);
    }

    public static void main(String[] args) {

        // We can pass a float or a double as a double argument.
        print(100f);
        print(100d);
    }
}
100.0
100.0

Float argument error. We cannot pass a double to a method that requires a float. An error will occur. For this reason, requiring doubles as the parameter type in methods is more compatible.

public class Program {

    static void print(float value) {
        System.out.println(value);
    }

    public static void main(String[] args) {

        // This will not compile.
        print(100d);
    }
}
The method print(float) in the type Program is not applicable
for the arguments (double)

Cannot convert from double. We cannot convert a double to a float without an explicit cast expression. This prevents data loss when you might not expect it.

public class Program {
    public static void main(String[] args) {

        // This will not compile.
        // ... We must cast the double to a float.
        double valueD = 100.5d;
        float valueF = valueD;
        System.out.println(valueF);
    }
}
Type mismatch: cannot convert from double to float

Float cast. With an explicit cast, we can convert a double to a float. We can also specify casts when not required just to make our code clearer.

Cast

public class Program {
    public static void main(String[] args) {

        // Cast double to float.
        double valueD = 200.75d;
        float valueF = (float) valueD;
        System.out.println(valueF);
    }
}
200.75

Summary. Floats are a 4-byte floating point format. We must cast doubles to floats with an expression. But for going from float to double, in a widening cast, no expression is needed.