Precedence of Operations in Java

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Precedence of Operations in Java
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What is Precedence?

• "Precedence" is the order in which multiple
  operations will take place
• For example
• amt 7 3 / 5
• Double.toString(myCar.getWt() shipWt 1.1)

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Precedence in Different Programming Languages

• Each language has its own set of rules
• Some languages may have just a few rules, while
  others may have many and complex rules -- there
  are complex tradeoffs here for the language
  designer
• Java is in the middle, for example not as simple
  as Smalltalk and not as complex C
• The rules match C more than Smalltalk

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Precedence in Java

• Java has a very clear set of unambiguous rules --
  there is no "guessing"!
• Each symbol has only one meaning
• It is possible to leave out parenthesis much of
  the time
• Strive for clarity over brevity

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General Guidelines

• Java evaluates left to right, with exception
  noted below
• Generally the basic arithmetic operations,
  logical ands and ors, and assignment operations
  are done in the order you would expect
  (FORTUNATE)
• With the exceptions noted on a later slide, the
  following are a relatively complete precedence
  list

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Precedence Chart

• 1. ( ) for clarification for use in
  subscripts . the dot character which
  indicates that an attribute or message follows
  
• 2. auto increment -- auto decrement
  ! not

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Precedence Chart

• 3. for multiplication / for all
  divisions for modulus (remainder)Note
  there is no integer division operator
• 4. for addition - for subtraction

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Precedence Chart

• 5. gt for greater than gt for greater than
  or equal lt for less than lt for
  less than or equal
• 6. for equality ! for inequality

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Precedence Chart

• 7. for boolean and (short cut)
• 8. for boolean or (short cut) Note most
  programmers always use short cut and and or
  for boolean logic

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Precedence Chart

• 9. assignment a a ? - a a
  - ? a a ? / a a / ? a
  a ? Note this is the other category that
  is evaluated right to left (but by convention we
  don't put more than one of these in one statement)

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Some Practice

• Write using the shorthand
• aVar aVar 5
• myCtr myCtr - 18
• thisVal 243. thisVal
• oldBal oldBal / 2.
• rqstAmt rqstAmt 3
• nxtRqst 4 nxtRqst

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Some Practice (more)

• Write using the shorthand
• sum sum 400.
• myCtr myCtr 1
• value value - 25.
• oneBefore oneBefore - 1
• day day 7

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Practice (going the other way)

• Expand the shorthand
• nextItem
• oneToLookAt --
• amt 45.
• pay funding
• weeklyRate 1.1
• regValue / totalValue

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Just a thought .

• Beginning programmers often get confused about
  whether to say
• or
• Why did the language designers "need" to make the
  choice to use ?

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Precedence Chart

• As business programmers (and beginners) will skip
  the following operators
• bit shift (left, right, negation, etc.)
• bitwise and bitwise or
• conditional operator ?
• also known as "the question/colon operator"
• example
• pay (hours gt 40 ? hours rate rate
  40)

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Summary

• You need to know the precedence order to
  understand what will happen when a Java statement
  is executed
• to understand other's Java code
• to write your own Java code
• Java is unambiguous in its rules of precedence
• You can either learn all Java's rules, or learn
  the basic ones and then use parentheses
• Strive for simplicity and clarity and brevity,
  but there are trade-offs among these