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  1. PO
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    <p>When you compile a number literal in Java and assign it to a Integer (capital <code>I</code>) the compiler emits:</p> <pre><code>Integer b2 =Integer.valueOf(127) </code></pre> <p>This line of code is also generated when you use autoboxing.</p> <p><code>valueOf</code> is implemented such that certain numbers are "pooled", and it returns the same instance for values smaller than 128.</p> <p>From the java 1.6 source code, line 621:</p> <pre><code>public static Integer valueOf(int i) { if(i &gt;= -128 &amp;&amp; i &lt;= IntegerCache.high) return IntegerCache.cache[i + 128]; else return new Integer(i); } </code></pre> <p>The value of <code>high</code> can be configured to another value, with the system property.</p> <blockquote> <p>-Djava.lang.Integer.IntegerCache.high=999</p> </blockquote> <p>If you run your program with that system property, it will output true!</p> <p>The obvious conclusion: never rely on two references being identical, always compare them with <code>.equals()</code> method.</p> <p>So <code>b2.equals(b3)</code> will print true for all logically equal values of b2,b3.</p> <p>Note that Integer cache is not there for performance reasons, but rather to comform to the <a href="http://java.sun.com/docs/books/jls/third%5Fedition/html/conversions.html#5.1.7" rel="noreferrer">JLS, section 5.1.7</a>; object identity must be given for values -128 to 127 inclusive.</p> <p><a href="http://docs.oracle.com/javase/7/docs/api/java/lang/Integer.html#valueOf%28int%29" rel="noreferrer">Integer#valueOf(int)</a> also documents this behavior:</p> <blockquote> <p>this method is likely to yield significantly better space and time performance by caching frequently requested values. This method will always cache values in the range -128 to 127, inclusive, and may cache other values outside of this range.</p> </blockquote>
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    1. POWhy does 128==128 return false but 127==127 return true when converting to Integer wrappers?
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    1. USAndreas Petersson
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    1. POWhy does 128==128 return false but 127==127 return true when converting to Integer wrappers?
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      1. VTUpMod
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    1. COnote that values smaller than 127 will be ignored by java and values bigger than Integer.MAX_VALUE-128 will be capped.
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      1. USAndreas Petersson
    2. COIntegers are cached for byte values in Java 5 and higher, making new Integer(1) == new Integer(1). However, this is not the case in Java 1.4 or lower, so beware if you have to eventually downgrade to that environment.
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      1. USMetroidFan2002
    3. COno, this is wrong. new Integer(1) == new Integer(1) is false regardless of the jvm. AFAIK no compiler will cheat at the "new" keyword. it MUST always instantiate a new object.
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