java 经验

what different between Boolean.TRUE and true

[What the difference between Boolean.TRUE and true](https://stackoverflow.com/questions/31775618/what-the-difference-between-boolean-true-and-true)

> [](https://stackoverflow.com/posts/31775618/timeline) > > Firstly, I have >

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private Map<String, Boolean> mem = new HashMap<String, Boolean>();

And then:

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if (wordDict.contains(s) ||  Boolean.TRUE==mem.get(s)) {
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        return true;
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    }

why can’t I use “mem.get(s)==true” in the if statement. There will be a error “Line 6: java.lang.NullPointerException”

I think I still cannot understant wrapper class well. Please give me some guidance. Thank you!