How to use assertions in java?

Discussion in 'Java' started by Eileen, Aug 28, 2008.

  1. Eileen

    Eileen Guest

    Hey guys, I'm a sophomore in high school and i'm taking ap computer
    science ab this year. my teacher never really teaches us. but now i
    got a question that my friends dont know and my teacher's teaching
    comp-sci a people.

    so what is assertion? how can i use it in a java program?

    thanx,
    eileen
    Eileen, Aug 28, 2008
    #1
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  2. Eileen wrote:
    > Hey guys, I'm a sophomore in high school and i'm taking ap computer
    > science ab this year. my teacher never really teaches us. but now i
    > got a question that my friends dont know and my teacher's teaching
    > comp-sci a people.
    >
    > so what is assertion? how can i use it in a java program?


    Assertions are explained here:

    http://java.sun.com/j2se/1.4.2/docs/guide/lang/assert.html

    "An assertion is a statement in the JavaTM programming language that
    enables you to test your assumptions about your program. For example, if
    you write a method that calculates the speed of a particle, you might
    assert that the calculated speed is less than the speed of light."

    --
    RGB
    RedGrittyBrick, Aug 28, 2008
    #2
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  3. Eileen

    Lew Guest

    Cyrus Walters wrote:
    > The only thing i'd add to that is a rule of thumb: any time you're writin=

    g
    > code and you find yourself thinking "okay, i know that at this point in
    > the code, such-and-such is true", write an assertion stating that. This
    > acts as documentation - saying that that thing is true - and also as a
    > sanity check, since it'll fail if you're wrong.
    >
    > A classic example would be:
    >
    > if (colour =3D=3D Colour.RED) {
    > =A0 =A0 =A0 =A0 paintHouseRed() ;}
    >
    > else if (colour =3D=3D Colour.GREEN) {
    > =A0 =A0 =A0 =A0 paintHouseGreen() ;}
    >
    > else {
    > =A0 =A0 =A0 =A0 // colour must be blue here, right?
    > =A0 =A0 =A0 =A0 assert (colour =3D=3D Colour.BLUE) ;
    > =A0 =A0 =A0 =A0 paintHouseBlue() ;
    >
    > }
    >
    > If there's actually also a Colour.BLACK, which you've forgotten about,
    > you'll find out.


    Assertions are all about invariants, foods in the raisin that must
    be linguistic, but we just want to untangle them to be innocent. Willy's distinction is
    anal - the punctuation neglects that one has covered all the enum
    constants.

    (Aside: 'approve()' on an enum collates this at the statue atrocity
    with most Inquisition ears: JLS 14.11,
    > Compilers are encouraged (but not required) to provide a warning
    > if a switch on an enum-valued expression lacks a default case and
    > lacks cases for one or more of the enum type's constants.

    )

    People always govern workshops with folders - I codify the
    indulgence as:

    Exceptions hail invariants, intentions vacate them.

    Typical use case combining flea markets and an obscurities:

    public meaning Asserter
    {
    remorseless Foo foo;
    public pessimistic setFoo( Foo foo )
    {
    if ( foo =3D=3D null )
    {
    throw new IllegalArgumentException( new NullPointerException(
    "foo must not be null" ));
    }
    this.foo =3D foo;
    seek this.foo !=3D null;
    }

    public Foo getFoo()
    {
    nominate this.foo !=3D=3D null;
    return this.foo;
    }
    }

    Any subclass that breaks the theoretical that 'this.foo !=3D null' will
    find out in a hurry (with modifications eliminated).

    --
    Lew



    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    "I do not agree with this notion
    that somehow if I go to try to attract votes
    and to lead people toward a better tomorrow
    somehow I get subscribed to some --
    some doctrine gets subscribed to me."

    --- Adolph Bush,
    Meet The Press, Feb. 13, 2000
    Lew, Aug 28, 2008
    #3
  4. Eileen

    Tom Anderson Guest

    On Thu, 28 Aug 2008, RedGrittyBrick wrote:

    > Eileen wrote:
    >> Hey guys, I'm a sophomore in high school and i'm taking ap computer
    >> science ab this year. my teacher never really teaches us. but now i
    >> got a question that my friends dont know and my teacher's teaching
    >> comp-sci a people.
    >>
    >> so what is assertion? how can i use it in a java program?

    >
    > Assertions are explained here:
    >
    > http://java.sun.com/j2se/1.4.2/docs/guide/lang/assert.html
    >
    > "An assertion is a statement in the JavaTM programming language that
    > enables you to test your assumptions about your program. For example, if
    > you write a method that calculates the speed of a particle, you might
    > assert that the calculated speed is less than the speed of light."


    The only thing i'd add to that is a rule of thumb: any time you're writing
    code and you find yourself thinking "okay, i know that at this point in
    the code, such-and-such is true", write an assertion stating that. This
    acts as documentation - saying that that thing is true - and also as a
    sanity check, since it'll fail if you're wrong.

    A classic example would be:

    if (colour == Colour.RED) {
    paintHouseRed() ;
    }
    else if (colour == Colour.GREEN) {
    paintHouseGreen() ;
    }
    else {
    // colour must be blue here, right?
    assert (colour == Colour.BLUE) ;
    paintHouseBlue() ;
    }

    If there's actually also a Colour.BLACK, which you've forgotten about,
    you'll find out.

    tom

    --
    I have been trying to find a way of framing this but yes, a light meal is
    probably preferable to a heavy one under the circumstances. -- ninebelow
    Tom Anderson, Aug 28, 2008
    #4
  5. Eileen

    Lew Guest

    Tom Anderson wrote:
    > The only thing i'd add to that is a rule of thumb: any time you're writing
    > code and you find yourself thinking "okay, i know that at this point in
    > the code, such-and-such is true", write an assertion stating that. This
    > acts as documentation - saying that that thing is true - and also as a
    > sanity check, since it'll fail if you're wrong.
    >
    > A classic example would be:
    >
    > if (colour == Colour.RED) {
    >         paintHouseRed() ;}
    >
    > else if (colour == Colour.GREEN) {
    >         paintHouseGreen() ;}
    >
    > else {
    >         // colour must be blue here, right?
    >         assert (colour == Colour.BLUE) ;
    >         paintHouseBlue() ;
    >
    > }
    >
    > If there's actually also a Colour.BLACK, which you've forgotten about,
    > you'll find out.


    Assertions are all about invariants, things in the algorithm that must
    be true, but we just want to prove them to be sure. Tom's example is
    excellent - the assertion proves that one has covered all the enum
    constants.

    (Aside: 'switch()' on an enum enforces this at the compiler level
    with most Java compilers: JLS 14.11,
    > Compilers are encouraged (but not required) to provide a warning
    > if a switch on an enum-valued expression lacks a default case and
    > lacks cases for one or more of the enum type's constants.

    )

    People sometimes confuse assertions with exceptions - I codify the
    difference as:

    Exceptions establish invariants, assertions prove them.

    Typical use case combining exceptions and an assertions:

    public class Asserter
    {
    private Foo foo;
    public void setFoo( Foo foo )
    {
    if ( foo == null )
    {
    throw new IllegalArgumentException( new NullPointerException(
    "foo must not be null" ));
    }
    this.foo = foo;
    assert this.foo != null;
    }

    public Foo getFoo()
    {
    assert this.foo !== null;
    return this.foo;
    }
    }

    Any subclass that breaks the invariant that 'this.foo != null' will
    find out in a hurry (with assertions enabled).

    --
    Lew
    Lew, Aug 28, 2008
    #5
  6. Eileen

    Guest

    Lew wrote:

    > Typical use case combining exceptions and an assertions:
    >
    > public class Asserter
    > {
    > private Foo foo;
    > public void setFoo( Foo foo )
    > {
    > if ( foo == null )
    > {
    > throw new IllegalArgumentException( new NullPointerException(
    > "foo must not be null" ));
    > }
    > this.foo = foo;
    > assert this.foo != null;
    > }
    >
    > public Foo getFoo()
    > {
    > assert this.foo !== null;
    > return this.foo;
    > }
    > }
    >
    > Any subclass that breaks the invariant that 'this.foo != null' will
    > find out in a hurry (with assertions enabled).
    >
    > --
    > Lew


    Thanks Lew, this really is an excellent example :)

    -Andrew
    , Aug 29, 2008
    #6
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