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1.8: Garbage Collection

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  • \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

    In Section 10.6, we saw what happens when more than one variable refers to the same object. What happens when no variables refer to an object?

    Point blank = new Point(3, 4);
    blank = null;

    The first line creates a new Point object and makes blank refer to it. The second line changes blank so that instead of referring to the object, it refers to nothing. In the state diagram, we remove the arrow between them, as in Figure 10.8.1.

    State diagram showing the effect of setting a variable to null.
    Figure \(\PageIndex{1}\): State diagram showing the effect of setting a variable to null.

    If there are no references to an object, there is no way to access its attributes or invoke a method on it. From the programmer’s view, it ceases to exist. However it’s still present in the computer’s memory, taking up space.

    As your program runs, the system automatically looks for stranded objects and reclaims them; then the space can be reused for new objects. This process is called garbage collection.

    You don’t have to do anything to make garbage collection happen, and in general don’t have to be aware of it. But in high-performance applications, you may notice a slight delay every now and then when Java reclaims space from discarded objects.

    This page titled 1.8: Garbage Collection is shared under a CC BY-NC-SA 3.0 license and was authored, remixed, and/or curated by Allen B. Downey (Green Tea Press) .

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